URL: http://www.tondering.dk/claus/calendar.html

                   FREQUENTLY ASKED QUESTIONS ABOUT
                              CALENDARS

Version 2.6 - 24 June 2003

Copyright and disclaimer
------------------------
        This document is Copyright (C) 2003 by Claus Tondering.
        E-mail: claus@tondering.dk.
        The document may be freely distributed, provided this
        copyright notice is included and no money is charged for
        the document.

        This document is provided "as is". No warranties are made as
        to its correctness.

Introduction
------------
        This is the calendar FAQ. Its purpose is to give an overview
        of the Christian, Hebrew, Persian, and Islamic calendars in
        common use. It will provide a historical background for the
        Christian calendar, plus an overview of the French
        Revolutionary calendar, the Maya calendar, and the Chinese
        calendar.

        Comments are very welcome. My e-mail address is given above.

        I would like to thank
              - Dr Monzur Ahmed of the University of Birmingham, UK,
              - Michael J Appel,
              - Jay Ball,
              - Tom Box,
              - Chris Carrier,
              - Simon Cassidy,
              - Claus Dobesch,
              - Leofranc Holford-Strevens,
              - David B. Kelley of the Hamamatsu University School of
                Medicine in Japan,
              - H. Koenig,
              - Graham Lewis,
              - Duncan MacGregor,
              - Marcos Montes,
              - James E. Morrison,
              - Waleed A. Muhanna of the Fisher College of Business,
                Columbus, Ohio, USA,
              - Stefan Potthast,
              - Yves Sagnier of the Centre d'Etudes de la Navigation
                Aerienne,
              - Paul Schlyter of the Swedish Amateur Astronomer's Society,
              - Dr John Stockton
        for their help with this document.

Changes in version 2.6
----------------------
        The names for the six additional days of the French
        revolutionary calendar have been corrected.
        A few minor corrections have been made.

Changes in version 2.5
----------------------
        A new section 2.12.4 has been added and the following sections
        renumbered.
        The description of the calculation of the Epact in section
        2.12.5 has been rewritten.
        Chapter 5 about the Persian calendar has been added and the
        following chapters renumbered.
        Section 10.5 now mentions a couple of calendar books.
        A few minor corrections have been made.

Writing dates and years
-----------------------
        Dates will be written in the British format (1 January)
        rather than the American format (January 1). Dates will
        occasionally be abbreviated: "1 Jan" rather than "1 January".

        Years before and after the "official" birth year of Christ
        will be written "45 BC" or "AD 1997", respectively. I prefer
        this notation over the secular "45 BCE" and "1997 CE"
        (See also section 2.13.4.)

The 'mod' operator
------------------
        Throughout this document the operator 'mod' will be used to
        signify the modulo or remainder operator. For example,
        17 mod 7=3 because the result of the division 17/7 is 2 with a
        remainder of 3.

The text in square brackets
---------------------------
        Square brackets [like this] identify information that I am
        unsure about and about which I would like more
        information. Please write me at claus@tondering.dk.


Index:
------

        1. What Astronomical Events Form the Basis of Calendars?
           1.1. What are equinoxes and solstices?
        2. The Christian Calendar
           2.1. What is the Julian calendar?
                2.1.1. What years are leap years?
                2.1.2. What consequences did the use of the Julian
                       calendar have?
           2.2. What is the Gregorian calendar?
                2.2.1. What years are leap years?
                2.2.2. Isn't there a 4000-year rule?
                2.2.3. Don't the Greek do it differently?
                2.2.4. When did country X change from the Julian to
                       the Gregorian calendar?
           2.3. What day is the leap day?
           2.4. What is the Solar Cycle?
           2.5. What day of the week was 2 August 1953?
           2.6. When can I reuse my 1992 calendar?
           2.7. What is the Roman calendar?
                2.7.1. How did the Romans number days?
           2.8. What is the proleptic calendar?
           2.9. Has the year always started on 1 January?
           2.10. Then what about leap years?
           2.11. What is the origin of the names of the months?
           2.12. What is Easter?
                2.12.1. When is Easter? (Short answer)
                2.12.2. When is Easter? (Long answer)
                2.12.3. What is the Golden Number?
                2.12.4. How does one calculate Easter then?
                2.12.5. What is the Epact?
                2.12.6. How does one calculate Gregorian Easter then?
                2.12.7. Isn't there a simpler way to calculate Easter?
                2.12.8. Is there a simple relationship between two
                       consecutive Easters?
                2.12.9. How frequently are the dates for Easter repeated?
                2.12.10. What about Greek Orthodox Easter?
                2.12.11. Did the Easter dates change in 2001?
           2.13. How does one count years?
                 2.13.1. How did Dionysius date Christ's birth?
                 2.13.2. Was Jesus born in the year 0?
                 2.13.3. When does the 3rd millennium start?
                 2.13.4. What do AD, BC, CE, and BCE stand for?
           2.14. What is the Indiction?
           2.15. What is the Julian period?
                 2.15.1. Is there a formula for calculating the Julian
                         day number?
                 2.15.2. What is the modified Julian day number?
                 2.15.3. What is the Lilian day number?
           2.16. What is the correct way to write dates?
        3. The Hebrew Calendar
           3.1. What does a Hebrew year look like?
           3.2. What years are leap years?
           3.3. What years are deficient, regular, and complete?
           3.4. When is New Year's day?
           3.5. When does a Hebrew day begin?
           3.6. When does a Hebrew year begin?
           3.7. When is the new moon?
           3.8. How does one count years?
        4. The Islamic Calendar
           4.1. What does an Islamic year look like?
           4.2. So you can't print an Islamic calendar in advance?
           4.3. How does one count years?
           4.4. When will the Islamic calendar overtake the Gregorian
                calendar?
           4.5. Doesn't Saudi Arabia have special rules?
        5. The Persian Calendar
           5.1. What does a Persian year look like?
           5.2. When does the Persian year begin?
           5.3. How does one count years?
           5.4. What years are leap years?
        6. The Week
           6.1. What is the origin of the 7-day week?
           6.2. What do the names of the days of the week mean?
           6.3. What is the system behind the planetary day names?
           6.4. Has the 7-day week cycle ever been interrupted?
           6.5. Which day is the day of rest?
           6.6. What is the first day of the week?
           6.7. What is the week number?
           6.8. How can I calculate the week number?
           6.9. Do weeks of different lengths exist?
        7. The French Revolutionary Calendar
           7.1. What does a Republican year look like?
           7.2. How does one count years?
           7.3. What years are leap years?
           7.4. How does one convert a Republican date to a Gregorian one?
        8. The Maya Calendar
           8.1. What is the Long Count?
                8.1.1. When did the Long Count start?
           8.2. What is the Tzolkin?
                8.2.1. When did the Tzolkin start?
           8.3. What is the Haab?
                8.3.1. When did the Haab start?
           8.4. Did the Mayas think a year was 365 days?
        9. The Chinese Calendar
           9.1. What does the Chinese year look like?
           9.2. What years are leap years?
           9.3. How does one count years?
           9.4. What is the current year in the Chinese calendar?
        10. Frequently Asked Questions about this FAQ
           10.1. Why doesn't the FAQ describe calendar X?
           10.2. Why doesn't the FAQ contain information X?
           10.3. Why don't you reply to my e-mail?
           10.4. How do I know that I can trust your information?
           10.5. Can you recommend any good books about calendars?
           10.6. Do you know a web site where I can find information
                 about X?
        11. Date


1. What Astronomical Events Form the Basis of Calendars?
--------------------------------------------------------

Calendars are normally based on astronomical events, and the two most
important astronomical objects are the sun and the moon. Their cycles
are very important in the construction and understanding of calendars.

Our concept of a year is based on the earth's motion around the sun.
The time from one fixed point, such as a solstice or equinox, to the
next is called a "tropical year". Its length is currently 365.242190
days, but it varies. Around 1900 its length was 365.242196 days, and
around 2100 it will be 365.242184 days. (This definition of the
tropical year is not quite accurate, see section 1.1 for more
details.)

Our concept of a month is based on the moon's motion around the earth,
although this connection has been broken in the calendar commonly used
now. The time from one new moon to the next is called a "synodic
month", and its length is currently 29.5305889 days, but it
varies. Around 1900 its length was 29.5305886 days, and around 2100 it
will be 29.5305891 days.

Note that these numbers are averages. The actual length of a
particular year may vary by several minutes due to the influence of
the gravitational force from other planets. Similarly, the time
between two new moons may vary by several hours due to a number of
factors, including changes in the gravitational force from the sun,
and the moon's orbital inclination.

It is unfortunate that the length of the tropical year is not a
multiple of the length of the synodic month. This means that with 12
months per year, the relationship between our month and the moon
cannot be maintained.

However, 19 tropical years is 234.997 synodic months, which is very
close to an integer. So every 19 years the phases of the moon fall on
the same dates (if it were not for the skewness introduced by leap
years). 19 years is called a Metonic cycle (after Meton, an astronomer
from Athens in the 5th century BC).

So, to summarise: There are three important numbers to note:
A tropical year is 365.24219 days.
A synodic month is 29.53059 days.
19 tropical years is close to an integral number of synodic months.

The Christian calendar is based on the motion of the earth around the
sun, while the months retain no connection with the motion of the moon.

On the other hand, the Islamic calendar is based on the motion of the
moon, while the year has no connection with the motion of the earth
around the sun.

Finally, the Hebrew calendar combines both, in that its years are
linked to the motion of the earth around the sun, and its months are
linked to the motion of the moon.


1.1. What are equinoxes and solstices?
--------------------------------------

Equinoxes and solstices are frequently used as anchor points for
calendars. For people in the northern hemisphere:

- Winter solstice is the time in December when the sun reaches its
  southernmost latitude. At this time we have the shortest day. The
  date is near 21 December.

- Summer solstice is the time in June when the sun reaches its
  northernmost latitude. At this time we have the longest day. The
  date is near 21 June.

- Vernal equinox is the time in March when the sun passes the equator
  moving from the southern to the northern hemisphere. Day and night
  have approximately the same length. The date is near 20 March.

- Autumnal equinox is the time in September when the sun passes the
  equator moving from the northern to the southern hemisphere. Day and
  night have approximately the same length. The date is near
  22 September.

For people in the southern hemisphere these events are shifted half a
year.

The astronomical "tropical year" is frequently defined as the time
between, say, two vernal equinoxes, but this is not actually true.
Currently the time between two vernal equinoxes is slightly greater
than the tropical year. The reason is that the earth's position in its
orbit at the time of solstices and equinoxes shifts slightly each year
(taking approximately 21,000 years to move all the way around the
orbit). This, combined with the fact that the earth's orbit is not
completely circular, causes the equinoxes and solstices to shift with
respect to each other.

The astronomer's mean tropical year is really a somewhat artificial
average of the period between the time when the sun is in any given
position in the sky with respect to the equinoxes and the next time
the sun is in the same position.


2. The Christian Calendar
-------------------------

The "Christian calendar" is the term traditionally used to designate
the calendar commonly in use, although its connection with
Christianity is highly debatable.

The Christian calendar has years of 365 or 366 days. It is divided into
12 months that have no relationship to the motion of the moon. In
parallel with this system, the concept of "weeks" groups the days in
sets of 7.

Two main versions of the Christian calendar have existed in recent
times: The Julian calendar and the Gregorian calendar. The difference
between them lies in the way they approximate the length of the
tropical year and their rules for calculating Easter.


2.1. What is the Julian calendar?
---------------------------------

The Julian calendar was introduced by Julius Caesar in 45 BC. It was
in common use until the 1500s, when countries started changing to the
Gregorian calendar (section 2.2). However, some countries (for
example, Greece and Russia) used it into the 1900s, and the Orthodox
church in Russia still uses it, as do some other Orthodox churches.

In the Julian calendar, the tropical year is approximated as 365 1/4
days = 365.25 days. This gives an error of 1 day in approximately 128
years.

The approximation 365 1/4 is achieved by having 1 leap year every 4
years.


2.1.1. What years are leap years?
---------------------------------

The Julian calendar has 1 leap year every 4 years:

        Every year divisible by 4 is a leap year.

However, the 4-year rule was not followed in the first years after the
introduction of the Julian calendar in 45 BC. Due to a counting error,
every 3rd year was a leap year in the first years of this calendar's
existence. The leap years were:

        45 BC(?), 42 BC, 39 BC, 36 BC, 33 BC, 30 BC,
        27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC,
        AD 8, AD 12, and every 4th year from then on.

Authorities disagree about whether 45 BC was a leap year or not.

There were no leap years between 9 BC and AD 8 (or, according to some
authorities, between 12 BC and AD 4). This period without leap years
was decreed by emperor Augustus in order to make up for the surplus of
leap years introduced previously, and it earned him a place in the
calendar as the 8th month was named after him.

It is a curious fact that although the method of reckoning years after
the (official) birthyear of Christ was not introduced until the 6th
century, by some stroke of luck the Julian leap years coincide with
years of our Lord that are divisible by 4.


2.1.2. What consequences did the use of the Julian calendar have?
-----------------------------------------------------------------

The Julian calendar introduces an error of 1 day every 128 years. So
every 128 years the tropical year shifts one day backwards with
respect to the calendar. Furthermore, the method for calculating the
dates for Easter was inaccurate and needed to be refined.

In order to remedy this, two steps were necessary: 1) The Julian
calendar had to be replaced by something more adequate. 2) The extra
days that the Julian calendar had inserted had to be dropped.

The solution to problem 1) was the Gregorian calendar described in
section 2.2.

The solution to problem 2) depended on the fact that it was felt that
21 March was the proper day for vernal equinox (because 21 March was
the date for vernal equinox during the Council of Nicaea in AD
325). The Gregorian calendar was therefore calibrated to make that day
vernal equinox.

By 1582 vernal equinox had moved (1582-325)/128 days = approximately
10 days backwards. So 10 days had to be dropped.



2.2. What is the Gregorian calendar?
------------------------------------

The Gregorian calendar is the one commonly used today. It was proposed
by Aloysius Lilius, a physician from Naples, and adopted by Pope
Gregory XIII in accordance with instructions from the Council of Trent
(1545-1563) to correct for errors in the older Julian Calendar. It was
decreed by Pope Gregory XIII in a papal bull on 24 February 1582. This
bull is named "Inter Gravissimas" after its first two words.

In the Gregorian calendar, the tropical year is approximated as
365 97/400 days = 365.2425 days. Thus it takes approximately 3300
years for the tropical year to shift one day with respect to the
Gregorian calendar.

The approximation 365 97/400 is achieved by having 97 leap years
every 400 years.


2.2.1. What years are leap years?
---------------------------------

The Gregorian calendar has 97 leap years every 400 years:

        Every year divisible by 4 is a leap year.
        However, every year divisible by 100 is not a leap year.
        However, every year divisible by 400 is a leap year after all.

So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,
2000, and 2400 are leap years.

(Destruction of a myth: There are no double leap years, i.e. no
years with 367 days. See, however, the note on Sweden in section
2.2.4.)


2.2.2. Isn't there a 4000-year rule?
------------------------------------

It has been suggested (by the astronomer John Herschel (1792-1871)
among others) that a better approximation to the length of the
tropical year would be 365 969/4000 days = 365.24225 days. This would
dictate 969 leap years every 4000 years, rather than the 970 leap
years mandated by the Gregorian calendar. This could be achieved by
dropping one leap year from the Gregorian calendar every 4000 years,
which would make years divisible by 4000 non-leap years.

This rule has, however, not been officially adopted.


2.2.3. Don't the Greek do it differently?
-----------------------------------------

When the Orthodox church in Greece finally decided to switch to the
Gregorian calendar in the 1920s, they tried to improve on the
Gregorian leap year rules, replacing the "divisible by 400" rule by
the following:

        Every year which when divided by 900 leaves a remainder of 200
        or 600 is a leap year.

This makes 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2800 non-leap
years, whereas 2000, 2400, and 2900 are leap years. This will not
create a conflict with the rest of the world until the year 2800.

This rule gives 218 leap years every 900 years, which gives us an
average year of 365 218/900 days = 365.24222 days, which is certainly
more accurate than the official Gregorian number of 365.2425 days.

However, this rule is *not* official in Greece.


2.2.4. When did country X change from the Julian to the Gregorian calendar?
---------------------------------------------------------------------------

The papal bull of February 1582 decreed that 10 days should be dropped
from October 1582 so that 15 October should follow immediately after
4 October, and from then on the reformed calendar should be used.

This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
countries followed shortly after, but Protestant countries were
reluctant to change, and the Greek orthodox countries didn't change
until the start of the 1900s.



Changes in the 1500s required 10 days to be dropped.
Changes in the 1600s required 10 days to be dropped.
Changes in the 1700s required 11 days to be dropped.
Changes in the 1800s required 12 days to be dropped.
Changes in the 1900s required 13 days to be dropped.

(Exercise for the reader: Why is the error in the 1600s the same as
in the 1500s.)

The following list contains the dates for changes in a number of
countries. It is very strange that in many cases there seems to be
some doubt among authorities about what the correct days are.
Different sources give very different dates in some cases. The list
below does not include all the different opinions about when the
change took place.

Albania:        December 1912

Austria: Different regions on different dates
         Brixen, Salzburg and Tyrol:
                 5 Oct 1583 was followed by 16 Oct 1583
         Carinthia and Styria:
                14 Dec 1583 was followed by 25 Dec 1583
         See also Czechoslovakia and Hungary

Belgium: See the Netherlands

Bulgaria:       31 Mar 1916 was followed by 14 Apr 1916

Canada: Different areas changed at different times.
        Newfoundland and Hudson Bay coast:
                2 Sep 1752 was followed by 14 Sep 1752
        Mainland Nova Scotia:
                Gregorian 1605 - 13 Oct 1710
                Julian 2 Oct 1710 - 2 Sep 1752
                Gregorian since 14 Sep 1752
        Rest of Canada:
                Gregorian from first European settlement

China:          The Gregorian calendar replaced the Chinese calendar
                in 1912, but the Gregorian calendar was not used
                throughout the country until the communist revolution
                of 1949.
                               
Czechoslovakia (i.e. Bohemia and Moravia):
                 6 Jan 1584 was followed by 17 Jan 1584

Denmark (including Norway):
                18 Feb 1700 was followed by 1 Mar 1700

Egypt:          1875

Estonia:        31 Jan 1918 was followed by 14 Feb 1918

Finland: Then part of Sweden. (Note, however, that Finland later
                became part of Russia, which then still used the
                Julian calendar. The Gregorian calendar remained
                official in Finland, but some use of the Julian 
                calendar was made.)

France:         9 Dec 1582 was followed by 20 Dec 1582
                Alsace: 5 Feb 1682 was followed by 16 Feb 1682
                Lorraine: 16 Feb 1760 was followed by 28 Feb 1760
                Strasbourg: February 1682

Germany: Different states on different dates:
                Catholic states on various dates in 1583-1585
                Prussia: 22 Aug 1610 was followed by 2 Sep 1610
                Protestant states: 18 Feb 1700 was followed by 1 Mar 1700
                (Many local variations)

Great Britain and Dominions:
                2 Sep 1752 was followed by 14 Sep 1752

Greece:         [9 Mar 1924 was followed by 23 Mar 1924
                (Some sources say 1916 and 1920)]

Hungary:        21 Oct 1587 was followed by 1 Nov 1587

Ireland: See Great Britain

Italy:          4 Oct 1582 was followed by 15 Oct 1582

Japan:          The Gregorian calendar was introduced to supplement the
                traditional Japanese calendar on 1 Jan 1873.

Latvia:         During German occupation 1915 to 1918

Lithuania:      1915

Luxemburg:      14 Dec 1582 was followed by 25 Dec 1582

Netherlands (including Belgium):
        Zeeland, Brabrant, and the "Staten Generaal":
                14 Dec 1582 was followed by 25 Dec 1582
        Holland:
                1 Jan 1583 was followed by 12 Jan 1583   
        Limburg and the southern provinces (currently Belgium):
                   20 Dec 1582 was followed by 31 Dec 1582
                or
                   21 Dec 1582 was followed by 1 Jan 1583
        Groningen:
                10 Feb 1583 was followed by 21 Feb 1583
                Went back to Julian in the summer of 1594
                31 Dec 1700 was followed by 12 Jan 1701
        Gelderland:
                30 Jun 1700 was followed by 12 Jul 1700
        Utrecht and Overijssel:
                30 Nov 1700 was followed by 12 Dec 1700
        Friesland:
                31 Dec 1700 was followed by 12 Jan 1701
        Drenthe:
                30 Apr 1701 was followed by 12 May 1701

Norway: Then part of Denmark.

Poland:         4 Oct 1582 was followed by 15 Oct 1582

Portugal:       4 Oct 1582 was followed by 15 Oct 1582

Romania:        31 Mar 1919 was followed by 14 Apr 1919
                [The Greek Orthodox parts of the country may have
                changed later.]

Russia:         31 Jan 1918 was followed by 14 Feb 1918
                [In the eastern parts of the country the change may
                not have occurred until 1920.]

Scotland: Much confusion exists regarding Scotland's change. Different
          authorities disagree about whether Scotland changed together
          with the rest of Great Britain, or if they had changed
          earlier.

Spain:          4 Oct 1582 was followed by 15 Oct 1582

Sweden (including Finland):
                17 Feb 1753 was followed by 1 Mar 1753 (see note below)

Switzerland:
        Catholic cantons: 1583, 1584 or 1597
        Protestant cantons:
                31 Dec 1700 was followed by 12 Jan 1701
        (Many local variations)

Turkey:         Gregorian calendar introduced 1 Jan 1927

United States: Different areas changed at different times.
        Along the Eastern seaboard: With Great Britain in 1752.
        Mississippi valley: With France in 1582.
        Texas, Florida, California, Nevada, Arizona, New Mexico:
          With Spain in 1582
        Washington, Oregon: With Britain in 1752.
        Alaska: October 1867 when Alaska became part of the USA.

Wales: See Great Britain

Yugoslavia:     1919

Sweden has a curious history. Sweden decided to make a gradual change
from the Julian to the Gregorian calendar. By dropping every leap year
from 1700 through 1740 the eleven superfluous days would be omitted
and from 1 Mar 1740 they would be in sync with the Gregorian
calendar. (But in the meantime they would be in sync with nobody!)

So 1700 (which should have been a leap year in the Julian calendar)
was not a leap year in Sweden. However, by mistake 1704 and 1708
became leap years. This left Sweden out of synchronisation with both
the Julian and the Gregorian world, so they decided to go *back* to
the Julian calendar. In order to do this, they inserted an extra day
in 1712, making that year a double leap year! So in 1712, February had
30 days in Sweden.

Later, in 1753, Sweden changed to the Gregorian calendar by dropping 11
days like everyone else.


2.3. What day is the leap day?
------------------------------

It is 24 February!

Weird? Yes! The explanation is related to the Roman calendar and is
found in section 2.7.1.

From a numerical point of view, of course 29 February is the extra
day. But from the point of view of celebration of feast days, the
following correspondence between days in leap years and non-leap
years has traditionally been used:

        Non-leap year           Leap year
        -------------           ----------
        22 February             22 February
        23 February             23 February
                                24 February (extra day)
        24 February             25 February
        25 February             26 February
        26 February             27 February
        27 February             28 February
        28 February             29 February

For example, the feast of St. Leander has been celebrated on 27
February in non-leap years and on 28 February in leap years.

Many countries are gradually changing the leap day from the 24th to
the 29th. This affects countries such as Sweden and Austria that
celebrate "name days" (i.e. each day is associated with a name).


2.4. What is the Solar Cycle?
-----------------------------

In the Julian calendar the relationship between the days of the week
and the dates of the year is repeated in cycles of 28 years. In the
Gregorian calendar this is still true for periods that do not cross
years that are divisible by 100 but not by 400.

A period of 28 years is called a Solar Cycle. The "Solar Number" of a
year is found as:

        Solar Number = (year + 8) mod 28 + 1

In the Julian calendar there is a one-to-one relationship between the
Solar Number and the day on which a particular date falls.

(The leap year cycle of the Gregorian calendar is 400 years, which is
146,097 days, which curiously enough is a multiple of 7. So in the
Gregorian calendar the equivalent of the "Solar Cycle" would be 400
years, not 7*400=2800 years as one might be tempted to believe.)


2.5. What day of the week was 2 August 1953?
--------------------------------------------

To calculate the day on which a particular date falls, the following
algorithm may be used (the divisions are integer divisions, in which
remainders are discarded):

a = (14 - month) / 12
y = year - a
m = month + 12*a - 2
For Julian calendar: d = (5 + day + y + y/4 + (31*m)/12) mod 7
For Gregorian calendar: d = (day + y + y/4 - y/100 + y/400 + (31*m)/12) mod 7

The value of d is 0 for a Sunday, 1 for a Monday, 2 for a Tuesday, etc.

Example: On what day of the week was the author born?

My birthday is 2 August 1953 (Gregorian, of course).

a = (14 - 8) / 12 = 0
y = 1953 - 0 = 1953
m = 8 + 12*0 - 2 = 6
d = (2 + 1953 + 1953/4 - 1953/100 + 1953/400 + (31*6)/12) mod 7
  = (2 + 1953 +  488   -    19    +     4    +    15    ) mod 7
  = 2443 mod 7
  = 0

I was born on a Sunday.


2.6. When can I reuse my 1992 calendar?
---------------------------------------

Let us first assume that you are only interested in which dates fall
on which days of the week; you are not interested in the dates for
Easter and other irregular holidays.

Let us further confine ourselves to the years 1901-2099.

With these restrictions, the answer is as follows:

  - If year X is a leap year, you can reuse its calendar in year X+28.

  - If year X is the first year after a leap year, you can reuse its
    calendar in years X+6, X+17, and X+28.

  - If year X is the second year after a leap year, you can reuse its
    calendar in years X+11, X+17, and X+28.

  - If year X is the third year after a leap year, you can reuse its
    calendar in years X+11, X+22, and X+28.

Note that the expression X+28 occurs in all four items above. So you
can always reuse your calendar every 28 years.

But if you also want your calendar's indication of Easter and other
Christian holidays to be correct, the rules are far too complex to be
put to a simple formula. Sometimes calendars can be reused after just
six years. For example, the calendars for the years 1981 and 1987 are
identical, even when it comes to the date for Easter. But sometimes a
very long time can pass before a calendar can be reused; if you happen
to have a calendar from 1940, you won't be able to reuse it until the
year 5280!


2.7. What is the Roman calendar?
--------------------------------

Before Julius Caesar introduced the Julian calendar in 45 BC, the
Roman calendar was a mess, and much of our so-called "knowledge" about
it seems to be little more than guesswork.

Originally, the year started on 1 March and consisted of only 304 days
or 10 months (Martius, Aprilis, Maius, Junius, Quintilis, Sextilis,
September, October, November, and December). These 304 days were
followed by an unnamed and unnumbered winter period. The Roman king
Numa Pompilius (c. 715-673 BC, although his historicity is disputed)
allegedly introduced February and January (in that order) between
December and March, increasing the length of the year to 354 or 355
days. In 450 BC, February was moved to its current position between
January and March.

In order to make up for the lack of days in a year, an extra month,
Intercalaris or Mercedonius, (allegedly with 22 or 23 days though some
authorities dispute this) was introduced in some years. In an 8 year
period the length of the years were:
        1: 12 months or 355 days
        2: 13 months or 377 days
        3: 12 months or 355 days
        4: 13 months or 378 days
        5: 12 months or 355 days
        6: 13 months or 377 days
        7: 12 months or 355 days
        8: 13 months or 378 days
A total of 2930 days corresponding to a year of 366 1/4 days. This
year was discovered to be too long, and therefore 7 days were later
dropped from the 8th year, yielding 365.375 days per year.

This is all theory. In practice it was the duty of the priesthood to
keep track of the calendars, but they failed miserably, partly due to
ignorance, partly because they were bribed to make certain years long
and other years short. Furthermore, leap years were considered unlucky
and were therefore avoided in time of crisis, such as the Second Punic
War.

In order to clean up this mess, Julius Caesar made his famous calendar
reform in 45 BC. We can make an educated guess about the length of the
months in the years 47 and 46 BC:

                47 BC          46 BC
January          29             29
February         28             24
Intercalaris                    27
March            31             31
April            29             29
May              31             31
June             29             29
Quintilis        31             31
Sextilis         29             29
September        29             29
October          31             31
November         29             29
Undecember                      33
Duodecember                     34
December         29             29
                ---            ---
Total           355            445

The length of the months from 45 BC onward were the same as the ones
we know today.

Occasionally one reads the following story:
        "Julius Caesar made all odd numbered months 31 days long, and
        all even numbered months 30 days long (with February having 29
        days in non-leap years). In 44 BC Quintilis was renamed
        'Julius' (July) in honour of Julius Caesar, and in 8 BC
        Sextilis became 'Augustus' in honour of emperor Augustus. When
        Augustus had a month named after him, he wanted his month to
        be a full 31 days long, so he removed a day from February and
        shifted the length of the other months so that August would
        have 31 days."
This story, however, has no basis in actual fact. It is a fabrication
possibly dating back to the 14th century.


2.7.1. How did the Romans number days?
--------------------------------------

The Romans didn't number the days sequentially from 1. Instead they
had three fixed points in each month:
        "Kalendae" (or "Calendae"), which was the first day of the month.
        "Idus", which was the 13th day of January, February, April,
                June, August, September, November, and December, or
                the 15th day of March, May, July, or October.
        "Nonae", which was the 9th day before Idus (counting Idus
                 itself as the 1st day).

The days between Kalendae and Nonae were called "the 5th day before
Nonae", "the 4th day before Nonae", "the 3rd day before Nonae", and
"the day before Nonae". (There was no "2nd day before Nonae". This was
because of the inclusive way of counting used by the Romans: To them,
Nonae itself was the first day, and thus "the 2nd day before" and "the
day before" would mean the same thing.)

Similarly, the days between Nonae and Idus were called "the Xth day
before Idus", and the days after Idus were called "the Xth day before
Kalendae (of the next month)".

Julius Caesar decreed that in leap years the "6th day before Kalendae
of March" should be doubled. So in contrast to our present system, in
which we introduce an extra date (29 February), the Romans had the
same date twice in leap years. The doubling of the 6th day before
Kalendae of March is the origin of the word "bissextile".  If we
create a list of equivalences between the Roman days and our current
days of February in a leap year, we get the following:

        7th day before Kalendae of March        23 February
        6th day before Kalendae of March        24 February
        6th day before Kalendae of March        25 February
        5th day before Kalendae of March        26 February
        4th day before Kalendae of March        27 February
        3rd day before Kalendae of March        28 February
        the day before Kalendae of March        29 February
        Kalendae of March                        1 March

You can see that the extra 6th day (going backwards) falls on what is
today 24 February. For this reason 24 February is still today
considered the "extra day" in leap years (see section 2.3). However,
at certain times in history the second 6th day (25 Feb) has been
considered the leap day.

Why did Caesar choose to double the 6th day before Kalendae of March?
It appears that the leap month Intercalaris/Mercedonius of the
pre-reform calendar was not placed after February, but inside it,
namely between the 7th and 6th day before Kalendae of March. It was
therefore natural to have the leap day in the same position.


2.8. What is the proleptic calendar?
------------------------------------

The Julian calendar was introduced in 45 BC, but when historians date
events prior to that year, they normally extend the Julian calendar
backward in time. This extended calendar is known as the "Julian
Proleptic Calendar".

Similarly, it is possible to extend the Gregorian calendar backward in
time before 1582. However, this "Gregorian Proleptic Calendar" is
rarely used.

If someone refers to, for example, 15 March 429 BC, they are probably
using the Julian proleptic calendar.

In the Julian proleptic calendar, year X BC is a leap year, if X-1 is
divisible by 4. This is the natural extension of the Julian leap year
rules.


2.9. Has the year always started on 1 January?
----------------------------------------------

For the man in the street, yes. When Julius Caesar introduced his
calendar in 45 BC, he made 1 January the start of the year, and it was
always the date on which the Solar Number and the Golden Number (see
section 2.12.3) were incremented.

However, the church didn't like the wild parties that took place at
the start of the new year, and in AD 567 the council of Tours declared
that having the year start on 1 January was an ancient mistake that
should be abolished.

Through the middle ages various New Year dates were used. If an
ancient document refers to year X, it may mean any of 7 different
periods in our present system:

- 1 Mar X to 28/29 Feb X+1
- 1 Jan X to 31 Dec X
- 1 Jan X-1 to 31 Dec X-1
- 25 Mar X-1 to 24 Mar X
- 25 Mar X to 24 Mar X+1
- Saturday before Easter X to Friday before Easter X+1
- 25 Dec X-1 to 24 Dec X

Choosing the right interpretation of a year number is difficult, so
much more as one country might use different systems for religious and
civil needs.

The Byzantine Empire used a year starting on 1 Sep, but they didn't
count years since the birth of Christ, instead they counted years
since the creation of the world which they dated to 1 September 5509 BC.

Since about 1600 most countries have used 1 January as the first day
of the year. Italy and England, however, did not make 1 January official
until around 1750.

In England (but not Scotland) three different years were used:
- The historical year, which started on 1 January.
- The liturgical year, which started on the first Sunday in advent.
- The civil year, which
        from the 7th to the 12th century started on 25 December,
        from the 12th century until 1751 started on 25 March,
        from 1752 started on 1 January.


2.10. Then what about leap years?
---------------------------------

If the year started on, for example, 1 March, two months later than
our present year, when was the leap day inserted?

[The following information is to the best of my knowledge true. If
anyone can confirm or refute it, please let me know.]

When it comes to determining if a year is a leap year, since AD 8 the
Julian calendar has always had 48 months between two leap days. So, in
a country using a year starting on 1 March, 1439 would have been a
leap year, because their February 1439 would correspond to February
1440 in the January-based reckoning.


2.11. What is the origin of the names of the months?
---------------------------------------------------

A lot of languages, including English, use month names based on Latin.
Their meaning is listed below. However, some languages (Czech and
Polish, for example) use quite different names.

January     Latin: Januarius. Named after the god Janus.
February    Latin: Februarius. Named after Februa, the purification
            festival.
March       Latin: Martius. Named after the god Mars.
April       Latin: Aprilis. Named either after the goddess Aphrodite or
            the Latin word "aperire", to open.
May         Latin: Maius. Probably named after the goddess Maia.
June        Latin: Junius. Probably named after the goddess Juno.
July        Latin: Julius. Named after Julius Caesar in 44 BC. Prior
            to that time its name was Quintilis from the word
            "quintus", fifth, because it was the 5th month in the old
            Roman calendar.
August      Latin: Augustus. Named after emperor Augustus in 8
            BC. Prior to that time the name was Sextilis from the
            word "sextus", sixth, because it was the 6th month in the
            old Roman calendar.
September   Latin: September. From the word "septem", seven, because
            it was the 7th month in the old Roman calendar.
October     Latin: October. From the word "octo", eight, because it
            was the 8th month in the old Roman calendar.
November    Latin: November. From the word "novem", nine, because it
            was the 9th month in the old Roman calendar.
December    Latin: December. From the word "decem", ten, because it
            was the 10th month in the old Roman calendar.


2.12. What is Easter?
---------------------

In the Christian world, Easter (and the days immediately preceding it)
is the celebration of the death and resurrection of Jesus in
(approximately) AD 30.


2.12.1. When is Easter? (Short answer)
--------------------------------------

Easter Sunday is the first Sunday after the first full moon after
vernal equinox.


2.12.2. When is Easter? (Long answer)
-------------------------------------

The calculation of Easter is complicated because it is linked to (an
inaccurate version of) the Hebrew calendar.

Jesus was crucified immediately before the Jewish Passover, which is a
celebration of the Exodus from Egypt under Moses. Celebration of
Passover started on the 14th or 15th day of the (spring) month of
Nisan. Jewish months start when the moon is new, therefore the 14th or
15th day of the month must be immediately after a full moon.

It was therefore decided to make Easter Sunday the first Sunday after
the first full moon after vernal equinox. Or more precisely: Easter
Sunday is the first Sunday after the *official* full moon on or after
the *official* vernal equinox.

The official vernal equinox is always 21 March.

The official full moon may differ from the *real* full moon by one or
two days.

(Note, however, that historically, some countries have used the *real*
(astronomical) full moon instead of the official one when calculating
Easter. This was the case, for example, of the German Protestant states,
which used the astronomical full moon in the years 1700-1776. A
similar practice was used Sweden in the years 1740-1844 and in Denmark
in the 1700s.)

The full moon that precedes Easter is called the Paschal full
moon. Two concepts play an important role when calculating the Paschal
full moon: The Golden Number and the Epact. They are described in the
following sections.

The following sections give details about how to calculate the date
for Easter. Note, however, that while the Julian calendar was in use,
it was customary to use tables rather than calculations to determine
Easter. The following sections do mention how to calculate Easter
under the Julian calendar, but the reader should be aware that this is
an attempt to express in formulas what was originally expressed in
tables. The formulas can be taken as a good indication of when Easter
was celebrated in the Western Church from approximately the 6th
century.


2.12.3. What is the Golden Number?
----------------------------------

Each year is associated with a Golden Number.

Considering that the relationship between the moon's phases and the
days of the year repeats itself every 19 years (as described in
section 1), it is natural to associate a number between 1 and 19
with each year. This number is the so-called Golden Number. It is
calculated thus:
        GoldenNumber = (year mod 19)+1

In years which have the same Golden Number, the new moon will fall on
(approximately) the same date. The Golden Number is sufficient to
calculate the Paschal full moon in the Julian calendar.


2.12.4. How does one calculate Easter then?
-------------------------------------------

Under the Julian calendar the method was simple. If you know the
Golden Number of the year, you can find the Paschal full moon in this
table:

   Golden                 Golden                 Golden
   Number   Full moon     Number   Full moon     Number   Full moon
   ------------------     ------------------     ------------------
     1       5 April         8     18 April        15      1 April
     2      25 March         9      7 April        16     21 March
     3      13 April        10     27 March        17      9 April
     4       2 April        11     15 April        18     29 March
     5      22 March        12      4 April        19     17 April
     6      10 April        13     24 March
     7      30 March        14     12 April

Easter Sunday is the first Sunday following the above full moon date.
If the full moon falls on a Sunday, Easter Sunday is the following
Sunday.

But under the Gregorian calendar, things became much more complicated.
One of the changes made in the Gregorian calendar reform was a
modification of the way Easter was calculated. There were two reasons
for this. First, the 19 year cycle of the phases of moon (the Metonic
cycle) was known not to be perfect. Secondly, the Metonic cycle fitted
the Gregorian calendar year worse than it fitted the Julian calendar
year.

It was therefore decided to base Easter calculations on the so-called
Epact.


2.12.5. What is the Epact?
--------------------------

Each year is associated with an Epact.

The Epact is a measure of the age of the moon (i.e. the number of days
that have passed since an "official" new moon) on a particular date.

In the Julian calendar, the Epact is the age of the moon on 22 March.
In the Gregorian calendar, the Epact is the age of the moon at the
start of the year.

The Epact is linked to the Golden Number in the following manner:

Under the Julian calendar, 19 years were assumed to be exactly an
integral number of synodic months, and the following relationship
exists between the Golden Number and the Epact:

        Epact = (11 * (GoldenNumber-1)) mod 30 

If this formula yields zero, the Epact is by convention frequently
designated by the symbol * and its value is said to be 30. Weird?
Maybe, but people didn't like the number zero in the old days.

Since there are only 19 possible golden numbers, the Epact can have
only 19 different values: 1, 3, 4, 6, 7, 9, 11, 12, 14, 15, 17, 18, 20,
22, 23, 25, 26, 28, and 30.


In the Gregorian calendar reform, some modifications were made to the
simple relationship between the Golden Number and the Epact.

In the Gregorian calendar the Epact should be calculated thus (the
divisions are integer divisions, in which remainders are discarded):
        
1) Use the Julian formula:
      JulianEpact = (11 * (GoldenNumber-1)) mod 30

2) Calculate the so-called "Solar Equation":
        S = (3*century)/4

   The Solar Equation is an expression of the difference between the
   Julian and the Gregorian calendar. The value of S increases by one
   in every century year that is not a leap year.

   (For the purpose of this calculation century=20 is used for the
   years 1900 through 1999, and similarly for other centuries,
   although this contradicts the rules in section 2.13.3.)

3) Calculate the so-called "Lunar Equation":
        L = (8*century + 5)/25

   The Lunar Equation is an expression of the difference between the
   Julian calendar and the Metonic cycle. The value of L increases by
   one 8 times every 2500 years.

4) Calculate the Gregorian epact thus:
        GregorianEpact = JulianEpact - S + L + 8

   The number 8 is a constant that calibrates the starting point of
   the Gregorian Epact so that it matches the actual age of the moon
   on new year's day. Actually, this constant should have been 9, but
   8 was probably chosen as a safety precaution; the calculation was
   known to be inaccurate, and the sentiment was that it was better to
   celebrate Easter too late than too early.

5) Add or subtract 30 until GregorianEpact lies between 1 and 30.

In the Gregorian calendar, the Epact can have any value from 1 to 30.

Example: What was the Epact for 1992?

GoldenNumber = 1992 mod 19 + 1 = 17
1) JulianEpact = (11 * (17-1)) mod 30 = 26
2) S = (3*20)/4 = 15
3) L = (8*20 + 5)/25 = 6
4) GregorianEpact = 26 - 15 + 6 + 8 = 25
5) No adjustment is necessary

The Epact for 1992 was 25.


2.12.6. How does one calculate Gregorian Easter then?
-----------------------------------------------------

Look up the Epact in this table to find the date for the Paschal full
moon:

  Epact   Full moon     Epact   Full moon     Epact   Full moon
  -----------------     -----------------     -----------------
     1    12 April        11     2 April        21    23 March
     2    11 April        12     1 April        22    22 March
     3    10 April        13    31 March        23    21 March
     4     9 April        14    30 March        24    18 April
     5     8 April        15    29 March        25    18 or 17 April
     6     7 April        16    28 March        26    17 April
     7     6 April        17    27 March        27    16 April
     8     5 April        18    26 March        28    15 April
     9     4 April        19    25 March        29    14 April
    10     3 April        20    24 March        30    13 April

Easter Sunday is the first Sunday following the above full moon date.
If the full moon falls on a Sunday, Easter Sunday is the following
Sunday.


An Epact of 25 requires special treatment, as it has two dates in the
above table. There are two equivalent methods for choosing the correct
full moon date:

A) Choose 18 April, unless the current century contains years with an
   epact of 24, in which case 17 April should be used.

B) If the Golden Number is > 11 choose 17 April, otherwise choose 18 April.

The proof that these two statements are equivalent is left as an
exercise to the reader. (The frustrated ones may contact me for the
proof.)

Example: When was Easter in 1992?

In the previous section we found that the Golden Number for 1992 was
17 and the Epact was 25. Looking in the table, we find that the
Paschal full moon was either 17 or 18 April. By rule B above, we
choose 17 April because the Golden Number > 11.

17 April 1992 was a Friday. Easter Sunday must therefore have been
19 April.


2.12.7. Isn't there a simpler way to calculate Easter?
------------------------------------------------------

This is an attempt to boil down the information given in the previous
sections (the divisions are integer divisions, in which remainders are
discarded):

G = year mod 19

For the Julian calendar:
    I = (19*G + 15) mod 30
    J = (year + year/4 + I) mod 7

For the Gregorian calendar:
    C = year/100
    H = (C - C/4 - (8*C+13)/25 + 19*G + 15) mod 30
    I = H - (H/28)*(1 - (29/(H + 1))*((21 - G)/11))
    J = (year + year/4 + I + 2 - C + C/4) mod 7

Thereafter, for both calendars:
L = I - J
EasterMonth = 3 + (L + 40)/44
EasterDay = L + 28 - 31*(EasterMonth/4)


This algorithm is based in part on the algorithm of Oudin (1940) as
quoted in "Explanatory Supplement to the Astronomical Almanac",
P. Kenneth Seidelmann, editor.

People who want to dig into the workings of this algorithm, may be
interested to know that
    G is the Golden Number-1
    H is 23-Epact (modulo 30)
    I is the number of days from 21 March to the Paschal full moon
    J is the weekday for the Paschal full moon (0=Sunday, 1=Monday,
      etc.)
    L is the number of days from 21 March to the Sunday on or before
      the Paschal full moon (a number between -6 and 28)


2.12.8. Is there a simple relationship between two consecutive Easters?
-----------------------------------------------------------------------

Suppose you know the Easter date of the current year, can you easily
find the Easter date in the next year? No, but you can make a
qualified guess.

If Easter Sunday in the current year falls on day X and the next year
is not a leap year, Easter Sunday of next year will fall on one of the
following days: X-15, X-8, X+13 (rare), or X+20.

If Easter Sunday in the current year falls on day X and the next year
is a leap year, Easter Sunday of next year will fall on one of the
following days: X-16, X-9, X+12 (extremely rare), or X+19. (The jump
X+12 occurs only once in the period 1800-2200, namely when going from
2075 to 2076.)

If you combine this knowledge with the fact that Easter Sunday never
falls before 22 March and never falls after 25 April, you can
narrow the possibilities down to two or three dates.


2.12.9. How frequently are the dates for Easter repeated?
---------------------------------------------------------

The sequence of Easter dates repeats itself every 532 years in the
Julian calendar. The number 532 is the product of the following
numbers:

        19 (the Metonic cycle or the cycle of the Golden Number)
        28 (the Solar cycle, see section 2.4)

The sequence of Easter dates repeats itself every 5,700,000 years in
the Gregorian calendar. Calculating this is not as simple as for the
Julian calendar, but the number 5,700,000 turns out to be the product
of the following numbers:

        19 (the Metonic cycle or the cycle of the Golden Number)
        400 (the Gregorian equivalent of the Solar cycle, see section 2.4)
        25 (the cycle used in step 3 when calculating the Epact)
        30 (the number of different Epact values)


2.12.10. What about Greek Orthodox Easter?
------------------------------------------

The Greek Orthodox Church does not always celebrate Easter on the same
day as the Catholic and Protestant countries. The reason is that the
Orthodox Church uses the Julian calendar when calculating Easter. This
is case even in the churches that otherwise use the Gregorian
calendar.

When the Greek Orthodox Church in 1923 decided to change to the
Gregorian calendar (or rather: a Revised Julian Calendar), they chose
to use the astronomical full moon as the basis for calculating Easter,
rather than the "official" full moon described in the previous
sections. And they chose the meridian of Jerusalem to serve as
definition of when a Sunday starts. However, except for some sporadic
use the 1920s, this system was never adopted in practice.


2.12.11. Did the Easter dates change in 2001?
---------------------------------------------

No.

At a meeting in Aleppo, Syria (5-10 March 1997), organised by the
World Council of Churches and the Middle East Council of Churches,
representatives of several churches and Christian world communions
suggested that the discrepancies between Easter calculations in the
Western and the Eastern churches could be resolved by adopting
astronomically accurate calculations of the vernal equinox and the
full moon, instead of using the algorithm presented in section 2.12.6.
The meridian of Jerusalem should be used for the astronomical
calculations.

The new method for calculating Easter should have taken effect from
the year 2001. In that year the Julian and Gregorian Easter dates
coincided (on 15 April Gregorian/2 April Julian), and it would
therefore be a reasonable starting point for the new system.

However, the Eastern churches (especially the Russian Orthodox Church)
are reluctant to change, having already experienced a schism in the
calendar question. So nothing will happen in the near future.

If the new system were introduced, churches using the Gregorian
calendar will hardly notice the change. Only once during the period
2001-2025 would these churches note a difference: In 2019 the
Gregorian method gives an Easter date of 21 April, but the proposed
new method gives 24 March.

Note that the new method makes an Easter date of 21 March possible.
This date was not possible under the Julian or Gregorian algorithms.
(Under the new method, Easter will fall on 21 March in the year 2877.
You're all invited to my house on that date!)


2.13. How does one count years?
-------------------------------

In about AD 523, the papal chancellor, Bonifatius, asked a monk by the
name of Dionysius Exiguus to devise a way to implement the rules from
the Nicean council (the so-called "Alexandrine Rules") for general
use.

Dionysius Exiguus (in English known as Denis the Little) was a monk
from Scythia, he was a canon in the Roman curia, and his assignment
was to prepare calculations of the dates of Easter. At that time it
was customary to count years since the reign of emperor Diocletian;
but in his calculations Dionysius chose to number the years since the
birth of Christ, rather than honour the persecutor Diocletian.

Dionysius (wrongly) fixed Jesus' birth with respect to Diocletian's
reign in such a manner that it falls on 25 December 753 AUC (ab urbe
condita, i.e. since the founding of Rome), thus making the current era
start with AD 1 on 1 January 754 AUC.

How Dionysius established the year of Christ's birth is not known (see
section 2.13.1 for a couple of theories). Jesus was born under the
reign of king Herod the Great, who died in 750 AUC, which means that
Jesus could have been born no later than that year. Dionysius'
calculations were disputed at a very early stage.

When people started dating years before 754 AUC using the term "Before
Christ", they let the year 1 BC immediately precede AD 1 with no
intervening year zero. 

Note, however, that astronomers frequently use another way of
numbering the years BC. Instead of 1 BC they use 0, instead of 2 BC
they use -1, instead of 3 BC they use -2, etc.

See also section 2.13.2.

It is sometimes claimed that it was the Venerable Bede (673-735) who
introduced BC dating. Although Bede seems to have used the term on at
least one occasion, it is generally believed that BC dates were not
used until the middle of the 17th century.

In this section I have used AD 1 = 754 AUC. This is the most likely
equivalence between the two systems. However, some authorities state
that AD 1 = 753 AUC or 755 AUC. This confusion is not a modern one, it
appears that even the Romans were in some doubt about how to count
the years since the founding of Rome.


2.13.1. How did Dionysius date Christ's birth?
----------------------------------------------

There are quite a few theories about this. And many of the theories
are presented as if they were indisputable historical fact.

Here are two theories that I personally consider likely:

1. According to the Gospel of Luke (3:1 & 3:23) Jesus was "about
   thirty years old" shortly after "the fifteenth year of the reign of
   Tiberius Caesar". Tiberius became emperor in AD 14. If you combine
   these numbers you reach a birthyear for Jesus that is strikingly
   close to the beginning of our year reckoning. This may have been
   the basis for Dionysius' calculations.

2. Dionysius' original task was to calculate an Easter table. In the
   Julian calendar, the dates for Easter repeat every 532 years (see
   section 2.12.9). The first year in Dionysius' Easter tables is AD
   532. Is it a coincidence that the number 532 appears twice here? Or
   did Dionysius perhaps fix Jesus' birthyear so that his own Easter
   tables would start exactly at the beginning of the second Easter
   cycle after Jesus' birth?


2.13.2. Was Jesus born in the year 0?
-------------------------------------

No.

There are two reasons for this:
        - There is no year 0.
        - Jesus was born before 4 BC.

The concept of a year "zero" is a modern myth (but a very popular one).
Roman numerals do not have a figure designating zero, and treating zero
as a number on an equal footing with other numbers was not common in
the 6th century when our present year reckoning was established by
Dionysius Exiguus (see section 2.13). Dionysius let the year AD 1
start one week after what he believed to be Jesus' birthday.

Therefore, AD 1 follows immediately after 1 BC with no intervening
year zero. So a person who was born in 10 BC and died in AD 10,
would have died at the age of 19, not 20.

Furthermore, Dionysius' calculations were wrong. The Gospel of
Matthew tells us that Jesus was born under the reign of king Herod the
Great, who died in 4 BC. It is likely that Jesus was actually born
around 7 BC. The date of his birth is unknown; it may or may not be 25
December.


2.13.3. When did the 3rd millennium start?
------------------------------------------

The first millennium started in AD 1, so the millennia are counted in
this manner:

1st millennium:    1-1000
2nd millennium: 1001-2000
3rd millennium: 2001-3000

Thus, the 3rd millennium and, similarly, the 21st century started on
1 Jan 2001.

This is the cause of some heated debate, especially since some
dictionaries and encyclopaedias say that a century starts in years
that end in 00. Furthermore, the change 1999/2000 is obviously much
more spectacular than the change 2000/2001.

Let me propose a few compromises:

Any 100-year period is a century. Therefore the period from 23 June 2004
to 22 June 2104 is a century. So please feel free to celebrate the
start of a century any day you like!

Although the 20th century started in 1901, the 1900s started in 1900.
Similarly, the 21st century started in 2001, but the 2000s started in
2000.


2.13.4. What do AD, BC, CE, and BCE stand for?
----------------------------------------------

Years before the birth of Christ are in English traditionally
identified using the abbreviation BC ("Before Christ").

Years after the birth of Christ are traditionally identified using the
Latin abbreviation AD ("Anno Domini", that is, "In the Year of the
Lord").

Some people, who want to avoid the reference to Christ that is implied
in these terms, prefer the abbreviations BCE ("Before the Common Era"
or "Before the Christian Era") and CE ("Common Era" or "Christian Era").


2.14. What is the Indiction?
----------------------------

The Indiction was used in the middle ages to specify the position of a
year in a 15 year taxation cycle. It was introduced by emperor
Constantine the Great on 1 September 312 and ceased to be used in
1806.

The Indiction may be calculated thus:
        Indiction = (year + 2) mod 15 + 1

The Indiction has no astronomical significance.

The Indiction did not always follow the calendar year. Three different
Indictions may be identified:

1) The Pontifical or Roman Indiction, which started on New Year's Day
   (being either 25 December, 1 January, or 25 March).
2) The Greek or Constantinopolitan Indiction, which started on 1 September.
3) The Imperial Indiction or Indiction of Constantine, which started
   on 24 September.


2.15. What is the Julian Period?
--------------------------------

The Julian period (and the Julian day number) must not be confused
with the Julian calendar. 

The French scholar Joseph Justus Scaliger (1540-1609) was interested
in assigning a positive number to every year without having to worry
about BC/AD. He invented what is today known as the "Julian Period".

The Julian Period probably takes its name from the Julian calendar,
although it has been claimed that it is named after Scaliger's father,
the Italian scholar Julius Caesar Scaliger (1484-1558).

Scaliger's Julian period starts on 1 January 4713 BC (Julian calendar)
and lasts for 7980 years. AD 2003 is thus year 6716 in the Julian
period. After 7980 years the number starts from 1 again.

Why 4713 BC and why 7980 years? Well, in 4713 BC the Indiction (see
section 2.14), the Golden Number (see section 2.12.3) and the Solar
Number (see section 2.4) were all 1. The next times this happens is
15*19*28=7980 years later, in AD 3268.

Astronomers have used the Julian period to assign a unique number to
every day since 1 January 4713 BC. This is the so-called Julian Day
(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC
to noon UTC on 2 January 4713 BC.

This means that at noon UTC on 1 January AD 2000, JD 2,451,545
started.

This can be calculated thus:
        From 4713 BC to AD 2000 there are 6712 years.
        In the Julian calendar, years have 365.25 days, so 6712 years
        correspond to 6712*365.25=2,451,558 days. Subtract from this
        the 13 days that the Gregorian calendar is ahead of the Julian
        calendar, and you get 2,451,545.

Often fractions of Julian day numbers are used, so that 1 January AD
2000 at 15:00 UTC is referred to as JD 2,451,545.125.

Note that some people use the term "Julian day number" to refer to any
numbering of days. NASA, for example, uses the term to denote the
number of days since 1 January of the current year.


2.15.1. Is there a formula for calculating the Julian day number?
-----------------------------------------------------------------

Try this one (the divisions are integer divisions, in which remainders
are discarded):

   a = (14-month)/12
   y = year+4800-a
   m = month + 12*a - 3

  For a date in the Gregorian calendar:
   JD = day + (153*m+2)/5 + y*365 + y/4 - y/100 + y/400 - 32045

  For a date in the Julian calendar:
   JD = day + (153*m+2)/5 + y*365 + y/4 - 32083


JD is the Julian day number that starts at noon UTC on the specified
date.

The algorithm works fine for AD dates. If you want to use it for BC
dates, you must first convert the BC year to a negative year (e.g.,
10 BC = -9). The algorithm works correctly for all dates after 4800 BC,
i.e. at least for all positive Julian day numbers.


To convert the other way (i.e., to convert a Julian day number, JD,
to a day, month, and year) these formulas can be used (again, the
divisions are integer divisions):

  For the Gregorian calendar:
     a = JD + 32044
     b = (4*a+3)/146097
     c = a - (b*146097)/4

  For the Julian calendar:
     b = 0
     c = JD + 32082

  Then, for both calendars:
     d = (4*c+3)/1461
     e = c - (1461*d)/4
     m = (5*e+2)/153

     day   = e - (153*m+2)/5 + 1
     month = m + 3 - 12*(m/10)
     year  = b*100 + d - 4800 + m/10


2.15.2. What is the modified Julian day number?
-----------------------------------------------

Sometimes a modified Julian day number (MJD) is used which is
2,400,000.5 less than the Julian day number. This brings the numbers
into a more manageable numeric range and makes the day numbers change
at midnight UTC rather than noon.

MJD 0 thus started on 17 Nov 1858 (Gregorian) at 00:00:00 UTC.


2.15.3. What is the Lilian day number?
--------------------------------------

The Lilian day number is similar to the Julian day number, except that
Lilian day number 1 started at midnight on the first day of the
Gregorian calendar, that is, 15 October 1582.

The Lilian day number is named after Aloysius Lilius mentioned in
section 2.2.


2.16. What is the correct way to write dates?
---------------------------------------------

The answer to this question depends on what you mean by "correct".
Different countries have different customs.

Most countries use a day-month-year format, such as:
   25.12.1998   25/12/1998   25/12-1998   25.XII.1998

In the U.S.A. a month-day-year format is common:
   12/25/1998   12-25-1998

International standard ISO-8601 mandates a year-month-day format,
namely either 1998-12-25 or 19981225. This format is gaining
popularity in some countries.

In all of these systems, the first two digits of the year are
frequently omitted:
   25.12.98   12/25/98   98-12-25   

This confusion leads to misunderstandings. What is 02-03-04? To most
people it is 2 Mar 2004; to an American it is 3 Feb 2004; and to a
person using the international standard it would be 4 Mar 2002.

If you want to be sure that people understand you, I recommend that
you
  * write the month with letters instead of numbers, and
  * write the years as 4-digit numbers.


3. The Hebrew Calendar
----------------------

The current definition of the Hebrew calendar is generally said to
have been set down by the Sanhedrin president Hillel II in
approximately AD 359. The original details of his calendar are,
however, uncertain.

The Hebrew calendar is used for religious purposes by Jews all over
the world, and it is the official calendar of Israel.

The Hebrew calendar is a combined solar/lunar calendar, in that it
strives to have its years coincide with the tropical year and its
months coincide with the synodic months. This is a complicated goal,
and the rules for the Hebrew calendar are correspondingly
fascinating.


3.1. What does a Hebrew year look like?
---------------------------------------

An ordinary (non-leap) year has 353, 354, or 355 days.
A leap year has 383, 384, or 385 days.
The three lengths of the years are termed, "deficient", "regular",
and "complete", respectively.

An ordinary year has 12 months, a leap year has 13 months.

Every month starts (approximately) on the day of a new moon.

The months and their lengths are:

          Length in a      Length in a     Length in a
Name      deficient year   regular year    complete year
-------   --------------   ------------    -------------
Tishri          30              30              30
Heshvan         29              29              30
Kislev          29              30              30
Tevet           29              29              29
Shevat          30              30              30
(Adar I         30              30              30)
Adar II         29              29              29
Nisan           30              30              30
Iyar            29              29              29
Sivan           30              30              30
Tammuz          29              29              29
Av              30              30              30
Elul            29              29              29
-------   --------------   ------------    -------------
Total:      353 or 383      354 or 384      355 or 385

The month Adar I is only present in leap years. In non-leap years
Adar II is simply called "Adar".

Note that in a regular year the numbers 30 and 29 alternate; a
complete year is created by adding a day to Heshvan, whereas a
deficient year is created by removing a day from Kislev.

The alteration of 30 and 29 ensures that when the year starts with a
new moon, so does each month.


3.2. What years are leap years?
-------------------------------

A year is a leap year if the number 'year mod 19' is one of the
following: 0, 3, 6, 8, 11, 14, or 17.

The value for year in this formula is the "Anno Mundi" described in
section 3.8.


3.3. What years are deficient, regular, and complete?
-----------------------------------------------------

That is the wrong question to ask. The correct question to ask is: When
does a Hebrew year begin? Once you have answered that question (see
section 3.6), the length of the year is the number of days between
1 Tishri in one year and 1 Tishri in the following year.


3.4. When is New Year's day?
----------------------------

That depends. Jews have 4 different days to choose from:

1 Tishri:  "Rosh HaShanah". This day is a celebration of the creation
           of the world and marks the start of a new calendar
           year. This will be the day we shall base our calculations on
           in the following sections.

15 Shevat: "Tu B'shevat". The new year for trees, when fruit tithes
           should be brought.

1 Nisan:   "New Year for Kings". Nisan is considered the first month,
           although it occurs 6 or 7 months after the start of the
           calendar year.

1 Elul:    "New Year for Animal Tithes (Taxes)".

Only the first two dates are celebrated nowadays.


3.5. When does a Hebrew day begin?
----------------------------------

A Hebrew-calendar day does not begin at midnight, but at either sunset
or when three medium-sized stars should be visible, depending on the
religious circumstance.

Sunset marks the start of the 12 night hours, whereas sunrise marks the
start of the 12 day hours. This means that night hours may be longer
or shorter than day hours, depending on the season.


3.6. When does a Hebrew year begin?
-----------------------------------

The first day of the calendary year, Rosh HaShanah, on 1 Tishri is
determined as follows:

1) The new year starts on the day of the new moon that occurs about
   354 days (or 384 days if the previous year was a leap year) after
   1 Tishri of the previous year

2) If the new moon occurs after noon on that day, delay the new year
   by one day. (Because in that case the new crescent moon will not be
   visible until the next day.)

3) If this would cause the new year to start on a Sunday, Wednesday,
   or Friday, delay it by one day. (Because we want to avoid that
   Yom Kippur (10 Tishri) falls on a Friday or Sunday, and that
   Hoshanah Rabba (21 Tishri) falls on a Sabbath (Saturday)).

4) If two consecutive years start 356 days apart (an illegal year
   length), delay the start of the first year by two days.

5) If two consecutive years start 382 days apart (an illegal year
   length), delay the start of the second year by one day.


Note: Rule 4 can only come into play if the first year was supposed
to start on a Tuesday. Therefore a two day delay is used rather that a
one day delay, as the year must not start on a Wednesday as stated in
rule 3.


3.7. When is the new moon?
--------------------------

A calculated new moon is used. In order to understand the
calculations, one must know that an hour is subdivided into 1080
"parts".

The calculations are as follows:

The new moon that started the year AM 1, occurred 5 hours and 204
parts after sunset (i.e. just before midnight on Julian date 6 October
3761 BC).

The new moon of any particular year is calculated by extrapolating
from this time, using a synodic month of 29 days 12 hours and 793
parts.

Note that 18:00 Jerusalem time (15:39 UTC) is used instead of sunset in 
all these calculations.


3.8. How does one count years?
------------------------------

Years are counted since the creation of the world, which is assumed to
have taken place in 3761 BC. In that year, AM 1 started (AM = Anno
Mundi = year of the world).

In the year AD 2003 we witness the start of Hebrew year AM 5764.


4. The Islamic Calendar
-----------------------

The Islamic calendar (or Hijri calendar) is a purely lunar
calendar. It contains 12 months that are based on the motion of the
moon, and because 12 synodic months is only 12*29.53=354.36 days, the
Islamic calendar is consistently shorter than a tropical year, and
therefore it shifts with respect to the Christian calendar.

The calendar is based on the Qur'an (Sura IX, 36-37) and its proper
observance is a sacred duty for Muslims.

The Islamic calendar is the official calendar in countries around the
Gulf, especially Saudi Arabia (but see section 4.5). But other Muslim
countries use the Gregorian calendar for civil purposes and only turn
to the Islamic calendar for religious purposes.


4.1. What does an Islamic year look like?
-----------------------------------------

The names of the 12 months that comprise the Islamic year are:

1. Muharram                          7. Rajab
2. Safar                             8. Sha'ban
3. Rabi' al-awwal (Rabi' I)          9. Ramadan
4. Rabi' al-thani (Rabi' II)        10. Shawwal
5. Jumada al-awwal (Jumada I)       11. Dhu al-Qi'dah
6. Jumada al-thani (Jumada II)      12. Dhu al-Hijjah

(Due to different transliterations of the Arabic alphabet, other
spellings of the months are possible.)

Each month starts when the lunar crescent is first seen (by an actual
human being) after a new moon.

Although new moons may be calculated quite precisely, the actual
visibility of the crescent is much more difficult to predict. It
depends on factors such as weather, the optical properties of the
atmosphere, and the location of the observer. It is therefore very
difficult to give accurate information in advance about when a new
month will start.

Furthermore, some Muslims depend on a local sighting of the moon,
whereas others depend on a sighting by authorities somewhere in the
Muslim world. Both are valid Islamic practices, but they may lead to
different starting days for the months.


4.2. So you can't print an Islamic calendar in advance?
-------------------------------------------------------

Not a reliable one. However, calendars are printed for planning
purposes, but such calendars are based on estimates of the visibility
of the lunar crescent, and the actual month may start a day earlier or
later than predicted in the printed calendar.

Different methods for estimating the calendars are used.

Some sources mention a crude system in which all odd numbered months
have 30 days and all even numbered months have 29 days with an extra
day added to the last month in "leap years" (a concept otherwise
unknown in the calendar). Leap years could then be years in which the
number 'year mod 30' is one of the following: 2, 5, 7, 10, 13, 16, 18,
21, 24, 26, or 29. (This is the algorithm used in the calendar program
of the Gnu Emacs editor.)

Such a calendar would give an average month length of 29.53056 days,
which is quite close to the synodic month of 29.53059 days, so *on the
average* it would be quite accurate, but in any given month it is
still just a rough estimate.

Better algorithms for estimating the visibility of the new moon have
been devised, and a number of computer programs with this purpose
exist.


4.3. How does one count years?
------------------------------

Years are counted since the Hijra, that is, Mohammed's emigration to
Medina in AD 622. On 16 July (Julian calendar) of that year, AH 1
started (AH = Anno Hegirae = year of the Hijra).

In the year AD 2003 we have witnessed the start of Islamic year AH 1424.

Note that although only 2003-622=1381 years have passed in the
Christian calendar, 1423 years have passed in the Islamic calendar,
because its year is consistently shorter (by about 11 days) than the
tropical year used by the Christian calendar.


4.4. When will the Islamic calendar overtake the Gregorian calendar?
--------------------------------------------------------------------

As the year in the Islamic calendar is about 11 days shorter than the
year in the Christian calendar, the Islamic years are slowly gaining
in on the Christian years. But it will be many years before the two
coincide. The 1st day of the 5th month of AD 20874 in the Gregorian
calendar will also be (approximately) the 1st day of the 5th month of
AH 20874 of the Islamic calendar.


4.5. Doesn't Saudi Arabia have special rules?
---------------------------------------------

Saudi Arabia doesn't rely on a visual sighting of the crescent moon to
fix the start of a new month. Instead they base their calendar on a
calculated astronomical moon.

Since 1999 (1420 AH) the rule has been as follows: On the 29th day of
an Islamic month, the times when the sun and the moon set are
compared. If the sun sets before the moon, the next day will be the
first of a new month; but if the moon sets before the sun, the next
day will be the last (30th) of the current month.

The times for the setting of the sun and the moon are calculated for
the coordinates of Mecca.


5. The Persian Calendar
-----------------------

The Persian calendar is a solar calendar with a starting point that
matches that of the Islamic calendar. Its origin can be traced back to
the 11th century when a group of astronomers (including the well-known
poet Omar Khayyam) created what is known as the Jalaali calendar.
However, a number of changes have been made to the calendar since
then.

The current calendar has been used in Iran since 1925 and in
Afghanistan since 1957. However, Afghanistan used the Islamic calendar
in the years 1999-2002.


5.1. What does a Persian year look like?
----------------------------------------

The names and lengths of the 12 months that comprise the Persian year
are:

1. Farvardin   (31 days)      7. Mehr    (30 days)  
2. Ordibehesht (31 days)      8. Aban    (30 days)  
3. Khordad     (31 days)      9. Azar    (30 days)  
4. Tir         (31 days)     10. Day     (30 days)  
5. Mordad      (31 days)     11. Bahman  (30 days)  
6. Shahrivar   (31 days)     12. Esfand  (29/30 days)  

(Due to different transliterations of the Persian alphabet, other
spellings of the months are possible.) In Afghanistan the months are
named differently.

The month of Esfand has 29 days in an ordinary year, 30 days in a leap
year.


5.2. When does the Persian year begin?
--------------------------------------

The Persian year starts at vernal equinox. If the astronomical vernal
equinox falls before noon (Tehran true time) on a particular day, then
that day is the first day of the year. If the astronomical vernal
equinox falls after noon, the following day is the first day of the
year.


5.3. How does one count years?
------------------------------

As in the Islamic calendar (section 4.3), years are counted since
Mohammed's emigration to Medina in AD 622. At vernal equinox of that
year, AP 1 started (AP = Anno Persico/Anno Persarum = Persian year).

Note that contrary to the Islamic calendar, the Persian calendar
counts solar years. In the year AD 2003 we have therefore witnessed
the start of Persian year 1382, but the start of Islamic year 1424.


5.4. What years are leap years?
-------------------------------

Since the Persian year is defined by the astronomical vernal equinox,
the answer is simply: Leap years are years in which there are 366 days
between two Persian new year's days.

However, basing the Persian calendar purely on an astronomical
observation of the vernal equinox is rejected by many, and a few
mathematical rules for determining the length of the year have been
suggested.

The most popular (and complex) of these is probably the following:

The calendar is divided into periods of 2820 years. These periods are
then divided into 88 cycles whose lengths follow this pattern:

29, 33, 33, 33, 29, 33, 33, 33, 29, 33, 33, 33, ...

This gives 2816 years. The total of 2820 years is achieved by
extending the last cycle by 4 years (for a total of 37 years).

If you number the years within each cycle starting with 0, then leap
years are the years that are divisible by 4, except that the year 0 is
not a leap year.

So within, say, a 29 year cycle, this is the leap year pattern:

Year            Year              Year              Year
 0  Ordinary      8  Leap          16  Leap          24  Leap
 1  Ordinary      9  Ordinary      17  Ordinary      25  Ordinary
 2  Ordinary     10  Ordinary      18  Ordinary      26  Ordinary
 3  Ordinary     11  Ordinary      19  Ordinary      27  Ordinary
 4  Leap         12  Leap          20  Leap          28  Leap 
 5  Ordinary     13  Ordinary      21  Ordinary
 6  Ordinary     14  Ordinary      22  Ordinary
 7  Ordinary     15  Ordinary      23  Ordinary

This gives a total of 683 leap years every 2820 years, which
corresponds to an average year length of 365 683/2820 = 365.24220
days. This is a better approximation to the tropical year than the
365.2425 days of the Gregorian calendar.

The current 2820 year period started in the year AP 475 (AD 1096).

This "mathematical" calendar currently coincides closely with the
purely astronomical calendar. In the years between AP 1244 and 1531
(AD 1865 and 2152) a discrepancy of one day is seen twice, namely in
AP 1404 and 1437 (starting at vernal equinox of AD 2025 and 2058).
However, outside this period, discrepancies are more frequent.


6. The Week
-----------

The Christian, the Hebrew, the Islamic, and the Persian calendars all
have a 7-day week.


6.1. What is the origin of the 7-day week?
------------------------------------------

Digging into the history of the 7-day week is a very complicated
matter. Authorities have very different opinions about the history of
the week, and they frequently present their speculations as if they
were indisputable facts. The only thing we seem to know for certain
about the origin of the 7-day week is that we know nothing for
certain.

The first pages of the Bible explain how God created the world in six
days and rested on the seventh. This seventh day became the Jewish
day of rest, the Sabbath, Saturday.

Extra-biblical locations sometimes mentioned as the birthplace of the
7-day week include: Babylon, Persia, and several others. The week was
known in Rome before the advent of Christianity.


6.2. What do the names of the days of the week mean?
----------------------------------------------------

An answer to this question is necessarily closely linked to the
language in question. Whereas most languages use the same names for
the months (with a few Slavonic languages as notable exceptions),
there is great variety in names that various languages use for the
days of the week. A few examples will be given here.

Except for the Sabbath, Jews simply number their week days.

A related method is partially used in Portuguese and Russian:

   English    Portuguese      Russian        Meaning of Russian name
   -------    ----------      -------        -----------------------
   Monday     segunda-feira   ponedelnik     After "do-nothing"
   Tuesday    terca-feira     vtornik        Second
   Wednesday  quarta-feira    sreda          Middle
   Thursday   quinta-feira    chetverg       Fourth
   Friday     sexta-feira     pyatnitsa      Fifth
   Saturday   sabado          subbota        Sabbath
   Sunday     domingo         voskresenye    Resurrection

Most Latin-based languages connect each day of the week with one of
the seven "planets" of the ancient times: Sun, Moon, Mercury, Venus,
Mars, Jupiter, and Saturn. French, for example, uses:

   English     French         "Planet"
   -------     ------         --------
   Monday      lundi          Moon
   Tuesday     mardi          Mars
   Wednesday   mercredi       Mercury
   Thursday    jeudi          Jupiter
   Friday      vendredi       Venus
   Saturday    samedi         Saturn
   Sunday      dimanche       (Sun)

The link with the sun has been broken in French, but Sunday was
called "dies solis" (day of the sun) in Latin.

It is interesting to note that also some Asiatic languages (for
example, Hindi, Japanese, and Korean) have a similar relationship
between the week days and the planets.

English has retained the original planets in the names for Saturday,
Sunday, and Monday. For the four other days, however, the names of
Anglo-Saxon or Nordic gods have replaced the Roman gods that gave
name to the planets. Thus, Tuesday is named after Tiw, Wednesday is
named after Woden, Thursday is named after Thor, and Friday is named
after Freya.


6.3. What is the system behind the planetary day names?
-------------------------------------------------------

As we saw in the previous section, the planets have given the week
days their names following this order:

   Moon, Mars, Mercury, Jupiter, Venus, Saturn, Sun

Why this particular order?

One theory goes as follows: If you order the "planets" according to
either their presumed distance from Earth (assuming the Earth to be
the centre of the universe) or their period of revolution around the
Earth, you arrive at this order:

   Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn

Now, assign (in reverse order) these planets to the hours of the day:

   1=Saturn, 2=Jupiter, 3=Mars, 4=Sun, 5=Venus, 6=Mercury, 7=Moon,
   8=Saturn, 9=Jupiter, etc., 23=Jupiter, 24=Mars

The next day will then continue where the old day left off:

  1=Sun, 2=Venus, etc., 23=Venus, 24=Mercury

And the next day will go

  1=Moon, 2=Saturn, etc.

If you look at the planet assigned to the first hour of each day, you
will note that the planets come in this order:

   Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus

This is exactly the order of the associated week days.

Coincidence? Maybe.


6.4. Has the 7-day week cycle ever been interrupted?
----------------------------------------------------

There is no record of the 7-day week cycle ever having been broken.
Calendar changes and reform have never interrupted the 7-day cycles.
It very likely that the week cycles have run uninterrupted at least
since the days of Moses (c. 1400 BC), possibly even longer.

Some sources claim that the ancient Jews used a calendar in which an
extra Sabbath was occasionally introduced. But this is probably not
true.


6.5. Which day is the day of rest?
----------------------------------

For the Jews, the Sabbath (Saturday) is the day of rest and
worship. On this day God rested after creating the world.

Most Christians have made Sunday their day of rest and worship,
because Jesus rose from the dead on a Sunday.

Muslims use Friday as their day of rest and worship. The Qur'an
calls Friday a holy day, the "king of days".


6.6. What is the first day of the week?
---------------------------------------

The Bible clearly makes Saturday (the Sabbath) the last day of the
week. Therefore it is common Jewish and Christian practice to regard
Sunday as the first day of the week (as is also evident from the
Portuguese names for the week days mentioned in section 6.2). However,
the fact that, for example, Russian uses the name "second" for
Tuesday, indicates that some nations regard Monday as the first day.

In international standard ISO-8601 the International Organization for
Standardization has decreed that Monday shall be the first day of the
week.


6.7. What is the week number?
-----------------------------

International standard ISO-8601 (mentioned in section 6.6) assigns a
number to each week of the year. A week that lies partly in one year
and partly in another is assigned a number in the year in which most
of its days lie. This means that

        Week 1 of any year is the week that contains 4 January,

or equivalently

        Week 1 of any year is the week that contains the first
        Thursday in January.

Most years have 52 weeks, but years that start on a Thursday and leap
years that start on a Wednesday have 53 weeks.

Note: This week numbering system is not commonly used in the United
States.


6.8. How can I calculate the week number?
-----------------------------------------

If you know the date, how do you calculate the corresponding week
number (as defined in ISO-8601)?

1) Using the formulas in section 2.15.1, calculate the Julian Day
   Number, J.

2) Perform the following calculations (in which the divisions are
   integer divisions in which the remainder is discarded):

   d4 = (J+31741 - (J mod 7)) mod 146097 mod 36524 mod 1461
   L  = d4/1460
   d1 = ((d4-L) mod 365) + L
   WeekNumber = d1/7+1

(I am very grateful to Stefan Potthast for this algorithm.)

Note that if the week number is 1, 52, or 53, the week may lie in two
different calendar years. However, the week is always considered to
lie in the year in which it is counted. Thus, 31 December of year X,
may belong to week 1 of year X+1; similarly 1 January of year X may
belong to week 52 or 53 or year X-1.

 
6.9. Do weeks of different lengths exist?
-----------------------------------------

If you define a "week" as a 7-day period, obviously the answer is
no. But if you define a "week" as a named interval that is greater
than a day and smaller than a month, the answer is yes.

The ancient Egyptians used a 10-day "week", as did the French
Revolutionary calendar (see section 7.1).

The Maya calendar uses a 13 and a 20-day "week" (see section 8.2).

The Soviet Union has used both a 5-day and a 6-day week. In 1929-30
the USSR gradually introduced a 5-day week. Every worker had one day
off every week, but there was no fixed day of rest. On 1 September
1931 this was replaced by a 6-day week with a fixed day of rest,
falling on the 6th, 12th, 18th, 24th, and 30th day of each month (1
March was used instead of the 30th day of February, and the last day
of months with 31 days was considered an extra working day outside
the normal 6-day week cycle). A return to the normal 7-day week was
decreed on 26 June 1940.


7. The French Revolutionary Calendar
------------------------------------

The French Revolutionary Calendar (or Republican Calendar) was
introduced in France on 24 November 1793 and abolished on 1 January
1806. It was used again briefly during the Paris Commune in 1871.


7.1. What does a Republican year look like?
-------------------------------------------

A year consists of 365 or 366 days, divided into 12 months of 30 days
each, followed by 5 or 6 additional days. The months were:

1. Vendemiaire           7. Germinal
2. Brumaire              8. Floreal
3. Frimaire              9. Prairial
4. Nivose               10. Messidor
5. Pluviose             11. Thermidor
6. Ventose              12. Fructidor

(The second e in Vendemiaire and the e in Floreal carry an acute
accent. The o's in Nivose, Pluviose, and Ventose carry a circumflex
accent.)

The year was not divided into weeks, instead each month was divided
into three "decades" of 10 days, of which the final day was a day of
rest. This was an attempt to de-Christianize the calendar, but it was
an unpopular move, because now there were 9 work days between each day
of rest, whereas the Gregorian Calendar had only 6 work days between
each Sunday.

The ten days of each decade were called, respectively, Primidi, Duodi,
Tridi, Quartidi, Quintidi, Sextidi, Septidi, Octidi, Nonidi, Decadi.

The 5 or 6 additional days followed the last day of Fructidor and were
called:
      1. Fete de la vertu (Celebration of virtue)
      2. Fete du genie (Celebration of genius)
      3. Fete du travail (Celebration of labour)
      4. Fete de l'opinion (Celebration of opinion)
      5. Fete des recompenses (Celebration of rewards)
      6. Jour de la revolution (Day of the revolution)  (the leap day)

Each year was supposed to start on autumnal equinox (around 22
September), but this created problems as will be seen in section 7.3.


7.2. How does one count years?
------------------------------

Years are counted since the establishment of the first French Republic
on 22 September 1792. That day became 1 Vendemiaire of the year 1 of
the Republic. (However, the Revolutionary Calendar was not introduced
until 24 November 1793.)


7.3. What years are leap years?
-------------------------------

Leap years were introduced to keep New Year's Day on autumnal
equinox. But this turned out to be difficult to handle, because
equinox is not completely simple to predict.

In fact, the first decree implementing the calendar (5 Oct 1793)
contained two contradictory rules, as it stated that:
  - the first day of each year would be that of the autumnal equinox
  - every 4th year would be a leap year

In practice, the first calendars were based on the equinoxial
condition.

To remove the confusion, a rule similar to the one used in the
Gregorian Calendar (including a 4000 year rule as described in section
2.2.2) was proposed by the calendar's author, Charles Rommes, but his
proposal ran into political problems.

In short, during the time when the French Revolutionary Calendar was
in use, the following years were leap years: 3, 7, and 11.


7.4. How does one convert a Republican date to a Gregorian one?
---------------------------------------------------------------

The following table lists the Gregorian date on which each year of the
Republic started:

Year  1: 22 Sep 1792            Year  8: 23 Sep 1799
Year  2: 22 Sep 1793            Year  9: 23 Sep 1800
Year  3: 22 Sep 1794            Year 10: 23 Sep 1801
Year  4: 23 Sep 1795            Year 11: 23 Sep 1802
Year  5: 22 Sep 1796            Year 12: 24 Sep 1803
Year  6: 22 Sep 1797            Year 13: 23 Sep 1804
Year  7: 22 Sep 1798            Year 14: 23 Sep 1805


8. The Maya Calendar
--------------------

(I am very grateful to Chris Carrier for providing most of the
information about the Maya calendar.)

Among their other accomplishments, the ancient Mayas invented a
calendar of remarkable accuracy and complexity. The Maya calendar was
adopted by the other Mesoamerican nations, such as the Aztecs and the
Toltec, which adopted the mechanics of the calendar unaltered but
changed the names of the days of the week and the months.

The Maya calendar uses three different dating systems in parallel, the
"Long Count", the "Tzolkin" (divine calendar), and the "Haab" (civil
calendar).  Of these, only the Haab has a direct relationship to the
length of the year.

A typical Mayan date looks like this: 12.18.16.2.6, 3 Cimi 4 Zotz.

12.18.16.2.6 is the Long Count date.
3 Cimi is the Tzolkin date.
4 Zotz is the Haab date.


8.1. What is the Long Count?
----------------------------

The Long Count is really a mixed base-20/base-18 representation of a
number, representing the number of days since the start of the Mayan
era. It is thus akin to the Julian Day Number (see section 2.15).

The basic unit is the "kin" (day), which is the last component of the
Long Count. Going from right to left the remaining components are:

   uinal  (1 uinal = 20 kin = 20 days)
   tun    (1 tun = 18 uinal = 360 days = approx. 1 year)
   katun  (1 katun = 20 tun = 7,200 days = approx. 20 years)
   baktun (1 baktun = 20 katun = 144,000 days = approx. 394 years)

The kin, tun, and katun are numbered from 0 to 19.
The uinal are numbered from 0 to 17.
The baktun are numbered from 1 to 13.

Although they are not part of the Long Count, the Mayas had names for
larger time spans. The following names are sometimes quoted, although
they are not ancient Maya terms:

   1 pictun = 20 baktun = 2,880,000 days = approx. 7885 years
   1 calabtun = 20 pictun = 57,600,000 days = approx. 158,000 years
   1 kinchiltun = 20 calabtun = 1,152,000,000 days = approx. 3 million years
   1 alautun = 20 kinchiltun = 23,040,000,000 days = approx. 63 million years

The alautun is probably the longest named period in any calendar.


8.1.1. When did the Long Count start?
-------------------------------------

Logically, the first date in the Long Count should be 0.0.0.0.0, but
as the baktun (the first component) are numbered from 1 to 13 rather
than 0 to 12, this first date is actually written 13.0.0.0.0.

The authorities disagree on what 13.0.0.0.0 corresponds to in our
calendar. I have come across three possible equivalences:

13.0.0.0.0 =  8 Sep 3114 BC (Julian) = 13 Aug 3114 BC (Gregorian)
13.0.0.0.0 =  6 Sep 3114 BC (Julian) = 11 Aug 3114 BC (Gregorian)
13.0.0.0.0 = 11 Nov 3374 BC (Julian) = 15 Oct 3374 BC (Gregorian)

Assuming one of the first two equivalences, the Long Count will again
reach 13.0.0.0.0 on 21 or 23 December AD 2012 - a not too distant future.

The date 13.0.0.0.0 may have been the Mayas' idea of the date of the
creation of the world.


8.2. What is the Tzolkin?
-------------------------

The Tzolkin date is a combination of two "week" lengths.

While our calendar uses a single week of seven days, the Mayan
calendar used two different lengths of week:
   - a numbered week of 13 days, in which the days were numbered from
     1 to 13
   - a named week of 20 days, in which the names of the days were:

        0. Ahau         5. Chicchan     10. Oc          15. Men
        1. Imix         6. Cimi         11. Chuen       16. Cib
        2. Ik           7. Manik        12. Eb          17. Caban
        3. Akbal        8. Lamat        13. Ben         18. Etznab
        4. Kan          9. Muluc        14. Ix          19. Caunac

As the named week is 20 days and the smallest Long Count digit is 20
days, there is synchrony between the two; if, for example, the last
digit of today's Long Count is 0, today must be Ahau; if it is 6, it
must be Cimi. Since the numbered and the named week were both "weeks",
each of their name/number change daily; therefore, the day after 3
Cimi is not 4 Cimi, but 4 Manik, and the day after that, 5 Lamat. The
next time Cimi rolls around, 20 days later, it will be 10 Cimi instead
of 3 Cimi. The next 3 Cimi will not occur until 260 (or 13*20) days
have passed. This 260-day cycle also had good-luck or bad-luck
associations connected with each day, and for this reason, it became
known as the "divinatory year."

The "years" of the Tzolkin calendar are not counted.


8.2.1. When did the Tzolkin start?
----------------------------------

Long Count 13.0.0.0.0 corresponds to 4 Ahau. The authorities agree on
this.


8.3. What is the Haab?
----------------------

The Haab was the civil calendar of the Mayas. It consisted of 18
"months" of 20 days each, followed by 5 extra days, known as
"Uayeb". This gives a year length of 365 days.

The names of the month were:
        1. Pop           7. Yaxkin      13. Mac
        2. Uo            8. Mol         14. Kankin
        3. Zip           9. Chen        15. Muan
        4. Zotz         10. Yax         16. Pax
        5. Tzec         11. Zac         17. Kayab
        6. Xul          12. Ceh         18. Cumku
        
In contrast to the Tzolkin dates, the Haab month names changed every
20 days instead of daily; so the day after 4 Zotz would be 5 Zotz,
followed by 6 Zotz ... up to 19 Zotz, which is followed by 0 Tzec.

The days of the month were numbered from 0 to 19. This use of a 0th
day of the month in a civil calendar is unique to the Maya system; it
is believed that the Mayas discovered the number zero, and the uses to
which it could be put, centuries before it was discovered in Europe or
Asia.

The Uayeb days acquired a very derogatory reputation for bad luck;
known as "days without names" or "days without souls," and were
observed as days of prayer and mourning. Fires were extinguished and
the population refrained from eating hot food. Anyone born on those
days was "doomed to a miserable life."

The years of the Haab calendar are not counted.

The length of the Tzolkin year was 260 days and the length of the Haab
year was 365 days. The smallest number that can be divided evenly by
260 and 365 is 18,980, or 365*52; this was known as the Calendar
Round. If a day is, for example, "4 Ahau 8 Cumku," the next day
falling on "4 Ahau 8 Cumku" would be 18,980 days or about 52 years
later.  Among the Aztec, the end of a Calendar Round was a time of
public panic as it was thought the world might be coming to an
end. When the Pleiades crossed the horizon on 4 Ahau 8 Cumku, they
knew the world had been granted another 52-year extension.


8.3.1. When did the Haab start?
-------------------------------

Long Count 13.0.0.0.0 corresponds to 8 Cumku. The authorities agree on
this.


8.4. Did the Mayas think a year was 365 days?
---------------------------------------------

Although there were only 365 days in the Haab year, the Mayas were
aware that a year is slightly longer than 365 days, and in fact, many
of the month-names are associated with the seasons; Yaxkin, for
example, means "new or strong sun" and, at the beginning of the Long
Count, 1 Yaxkin was the day after the winter solstice, when the sun
starts to shine for a longer period of time and higher in the
sky. When the Long Count was put into motion, it was started at
7.13.0.0.0, and 0 Yaxkin corresponded with Midwinter Day, as it did at
13.0.0.0.0 back in 3114 B.C. The available evidence indicates that the
Mayas estimated that a 365-day year precessed through all the seasons
twice in 7.13.0.0.0 or 1,101,600 days.

We can therefore derive a value for the Mayan estimate of the year by
dividing 1,101,600 by 365, subtracting 2, and taking that number and
dividing 1,101,600 by the result, which gives us an answer of
365.242036 days, which is slightly more accurate than the 365.2425
days of the Gregorian calendar.

(This apparent accuracy could, however, be a simple coincidence. The
Mayas estimated that a 365-day year precessed through all the seasons
*twice* in 7.13.0.0.0 days. These numbers are only accurate to 2-3
digits. Suppose the 7.13.0.0.0 days had corresponded to 2.001 cycles
rather than 2 cycles of the 365-day year, would the Mayas have noticed?)


9. The Chinese Calendar
-----------------------

Although the People's Republic of China uses the Gregorian calendar
for civil purposes, a special Chinese calendar is used for determining
festivals. Various Chinese communities around the world also use this
calendar.

The beginnings of the Chinese calendar can be traced back to the 14th
century BC. Legend has it that the Emperor Huangdi invented the
calendar in 2637 BC.

The Chinese calendar is based on exact astronomical observations of
the longitude of the sun and the phases of the moon. This means that
principles of modern science have had an impact on the Chinese
calendar.

I can recommend visiting Helmer Aslaksen's web site at
http://www.chinesecalendar.net for more information about the Chinese
calendar.


9.1. What does the Chinese year look like?
------------------------------------------

The Chinese calendar - like the Hebrew - is a combined solar/lunar
calendar in that it strives to have its years coincide with the
tropical year and its months coincide with the synodic months. It is
not surprising that a few similarities exist between the Chinese and
the Hebrew calendar:

   * An ordinary year has 12 months, a leap year has 13 months.
   * An ordinary year has 353, 354, or 355 days, a leap year has 383,
     384, or 385 days.

When determining what a Chinese year looks like, one must make a
number of astronomical calculations:

First, determine the dates for the new moons. Here, a new moon is the
completely "black" moon (that is, when the moon is in conjunction with
the sun), not the first visible crescent used in the Islamic and
Hebrew calendars. The date of a new moon is the first day of a new
month.

Secondly, determine the dates when the sun's longitude is a multiple
of 30 degrees. (The sun's longitude is 0 at Vernal Equinox, 90 at
Summer Solstice, 180 at Autumnal Equinox, and 270 at Winter Solstice.)
These dates are called the "Principal Terms" and are used to determine
the number of each month:

Principal Term 1 occurs when the sun's longitude is 330 degrees.
Principal Term 2 occurs when the sun's longitude is 0 degrees.
Principal Term 3 occurs when the sun's longitude is 30 degrees.
etc.
Principal Term 11 occurs when the sun's longitude is 270 degrees.
Principal Term 12 occurs when the sun's longitude is 300 degrees.

Each month carries the number of the Principal Term that occurs in
that month.

In rare cases, a month may contain two Principal Terms; in this case
the months numbers may have to be shifted. Principal Term 11 (Winter
Solstice) must always fall in the 11th month.

All the astronomical calculations are carried out for the meridian 120
degrees east of Greenwich. This roughly corresponds to the east coast
of China.

Some variations in these rules are seen in various Chinese
communities.


9.2. What years are leap years?
-------------------------------

Leap years have 13 months. To determine if a year is a leap year,
calculate the number of new moons between the 11th month in one year
(i.e., the month containing the Winter Solstice) and the 11th month in
the following year. If there are 13 new moons from the start of the
11th month in the first year to the start of the 11th month in the
second year, a leap month must be inserted.

In leap years, at least one month does not contain a Principal Term.
The first such month is the leap month. It carries the same number as
the previous month, with the additional note that it is the leap
month.


9.3. How does one count years?
------------------------------

Unlike most other calendars, the Chinese calendar does not count years
in an infinite sequence. Instead years have names that are repeated
every 60 years.

(Historically, years used to be counted since the accession of an
emperor, but this was abolished after the 1911 revolution.)

Within each 60-year cycle, each year is assigned name consisting of
two components:

The first component is a "Celestial Stem":

        1. jia          6. ji
        2. yi           7. geng
        3. bing         8. xin
        4. ding         9. ren
        5. wu          10. gui

These words have no English equivalent.

The second component is a "Terrestrial Branch":

        1. zi (rat)             7. wu (horse)
        2. chou (ox)            8. wei (sheep)
        3. yin (tiger)          9. shen (monkey)
        4. mao (hare, rabbit)  10. you (rooster)
        5. chen (dragon)       11. xu (dog)
        6. si (snake)          12. hai (pig)

The names of the corresponding animals in the zodiac cycle of 12
animals are given in parentheses.

Each of the two components is used sequentially. Thus, the 1st year of
the 60-year cycle becomes jia-zi, the 2nd year is yi-chou, the 3rd
year is bing-yin, etc. When we reach the end of a component, we start
from the beginning: The 10th year is gui-you, the 11th year is jia-xu
(restarting the Celestial Stem), the 12th year is yi-hai, and the 13th
year is bing-zi (restarting the Terrestrial Branch). Finally, the 60th
year becomes gui-hai.

This way of naming years within a 60-year cycle goes back
approximately 2000 years. A similar naming of days and months has
fallen into disuse, but the date name is still listed in calendars.

It is customary to number the 60-year cycles since 2637 BC, when the
calendar was supposedly invented. In that year the first 60-year cycle
started.


9.4. What is the current year in the Chinese calendar?
------------------------------------------------------

The current 60-year cycle started on 2 Feb 1984. That date bears the name
bing-yin in the 60-day cycle, and the first month of that first year
bears the name gui-chou in the 60-month cycle.

This means that the year gui-wei, the 20th year in the 78th cycle,
started on 1 Feb 2003.



10. Frequently Asked Questions about this FAQ
---------------------------------------------

This chapter does not answer questions about calendars. Instead it
answers questions that I am often asked about this document.


10.1. Why doesn't the FAQ describe calendar X?
----------------------------------------------

I am frequently asked to add a chapter describing the Japanese
calendar, the Ethiopian calendar, the Hindu calendar, etc.

But I have to stop somewhere. I have discovered that the more calendars
I include in the FAQ, the more difficult it becomes to ensure that the
information given is correct. I want to work on the quality rather
than the quantity of information in this document. It is therefore not
likely that other calendars will be added in the near future.


10.2. Why doesn't the FAQ contain information X?
------------------------------------------------

Obviously, I cannot include everything. So I have to prioritize. The
things that are most likely to be omitted from the FAQ are:

- Information that is relevant to a single country only.
- Views that are controversial and not supported by recognized
  authorities.


10.3. Why don't you reply to my e-mail?
---------------------------------------

I try to reply to all the e-mail I receive. But occasionally the
amount of mail I receive is so large that I have to ignore some
letters. If this has caused your letter to be lost, I apologize.

But please don't let this stop you from writing to me. I enjoy
receiving letters, even if I can't answer them all.


10.4. How do I know that I can trust your information?
------------------------------------------------------

I have tried to be accurate in everything I have described. If you are
unsure about something that I write, I suggest that you try to verify
the information yourself. If you come across a recognized authority
that contradicts something that I've written, please let me know.


10.5. Can you recommend any good books about calendars?
-------------------------------------------------------

This is a big question because there are so many excellent books. At
this point I shall only recommend two books:

Edward M. Reingold & Nachum Dershowitz: "Calendrical Calculations.
The Millennium Edition". Cambridge University Press 2001.
ISBN 0-521-77752-6.
http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/index.html

  This book contains a lot of information about a huge number of
  calendars. As the title indicates, it has a strong emphasis on
  algorithms for calendrical calculations, so if you want to use your
  computer to compute calendars, this is a great book.

R. W. Bauer: "Calender for Aarene fra 601 til 2200". First published
in 1868. Reprinted 1993 by Dansk Historisk Faellesraad.
ISBN 87-7423-083-2

  Unfortunately, this book is in Danish, but if you can read the
  Scandinavian languages, this book will provide you with a wealth of
  information, despite its age. Its main strength is a huge collection
  of tables of various calendars. It does, however, only describe the
  Christian calendar.


10.6. Do you know a web site where I can find information about X?
------------------------------------------------------------------

Probably not.

Good places to start your calendar search include:

   http://www.calendarzone.com
   http://personal.ecu.edu/mccartyr/calendar-reform.html


11. Date
--------

This version 2.6 of this document was finished on

        Tuesday after the first Sunday after Trinity, the 24 of June
        anno ab Incarnatione Domini MMIII, indict. XI, epacta XVII,
        luna XXIV, anno post Margaretam Reginam Daniae natam LXIII, on
        the feast of Saint John the Baptist.
             
        The 24th day of Sivan, Anno Mundi 5763.

        The 23rd day of Rabi' al-thani, Anno Hegirae 1424.

        The 3rd day of Tir, Anno Persico 1382.

        The 25th day of the 5th month of the year gui-wei of the 78th
        cycle.

        Julian Day 2,452,815.
