NEWTZSET(3)                Library Functions Manual                NEWTZSET(3)

NAME
       tzset - initialize time conversion information

SYNOPSIS
       void tzset()

       cc ... -ltz

DESCRIPTION
       Tzset uses the value of the environment variable TZ to set time
       conversion information used by localtime.  If TZ does not appear in the
       environment, the best available approximation to local wall clock time,
       as specified by the tzfile(5)-format file localtime in the system time
       conversion information directory, is used by localtime.  If TZ appears
       in the environment but its value is a null string, Universal Time (UT)
       is used, with the abbreviation "UTC" and without leap second
       correction; please see newctime(3) for more about UT, UTC, and leap
       seconds.  If TZ appears in the environment and its value is not a null
       string:

              if the value begins with a colon, it is used as a pathname of a
              file from which to read the time conversion information;

              if the value does not begin with a colon, it is first used as
              the pathname of a file from which to read the time conversion
              information, and, if that file cannot be read, is used directly
              as a specification of the time conversion information.

       When TZ is used as a pathname, if it begins with a slash, it is used as
       an absolute pathname; otherwise, it is used as a pathname relative to a
       system time conversion information directory.  The file must be in the
       format specified in tzfile(5).

       When TZ is used directly as a specification of the time conversion
       information, it must have the following syntax (spaces inserted for
       clarity):

              stdoffset[dst[offset][,rule]]

       Where:

              std and dst    Three or more bytes that are the designation for
                             the standard (std) or summer (dst) time zone.
                             Only std is required; if dst is missing, then
                             summer time does not apply in this locale.
                             Upper- and lowercase letters are explicitly
                             allowed.  Any characters except a leading colon
                             (:), digits, comma (,), minus (-), plus (+), and
                             ASCII NUL are allowed.

              offset         Indicates the value one must add to the local
                             time to arrive at Coordinated Universal Time.
                             The offset has the form:

                                    hh[:mm[:ss]]

                             The minutes (mm) and seconds (ss) are optional.
                             The hour (hh) is required and may be a single
                             digit.  The offset following std is required.  If
                             no offset follows dst, summer time is assumed to
                             be one hour ahead of standard time.  One or more
                             digits may be used; the value is always
                             interpreted as a decimal number.  The hour must
                             be between zero and 24, and the minutes (and
                             seconds) -- if present -- between zero and 59.
                             If preceded by a ``-'', the time zone shall be
                             east of the Prime Meridian; otherwise it shall be
                             west (which may be indicated by an optional
                             preceding ``+'').

              rule           Indicates when to change to and back from summer
                             time.  The rule has the form:

                                    date/time,date/time

                             where the first date describes when the change
                             from standard to summer time occurs and the
                             second date describes when the change back
                             happens.  Each time field describes when, in
                             current local time, the change to the other time
                             is made.  As an extension to POSIX, daylight
                             saving is assumed to be in effect all year if it
                             begins January 1 at 00:00 and ends December 31 at
                             24:00 plus the difference between daylight saving
                             and standard time, leaving no room for standard
                             time in the calendar.

                             The format of date is one of the following:

                             Jn        The Julian day n (1 <= n <= 365).  Leap
                                       days are not counted; that is, in all
                                       years -- including leap years --
                                       February 28 is day 59 and March 1 is
                                       day 60.  It is impossible to explicitly
                                       refer to the occasional February 29.

                             n         The zero-based Julian day
                                       (0 <= n <= 365).  Leap days are
                                       counted, and it is possible to refer to
                                       February 29.

                             Mm.n.d    The d'th day (0 <= d <= 6) of week n of
                                       month m of the year (1 <= n <= 5,
                                       1 <= m <= 12, where week 5 means ``the
                                       last d day in month m'' which may occur
                                       in either the fourth or the fifth
                                       week).  Week 1 is the first week in
                                       which the d'th day occurs.  Day zero is
                                       Sunday.

                             The time has the same format as offset except
                             that POSIX does not allow a leading sign (``-''
                             or ``+'').  As an extension to POSIX, the hours
                             part of time can range from -167 through 167;
                             this allows for unusual rules such as "the
                             Saturday before the first Sunday of March".  The
                             default, if time is not given, is 02:00:00.

       Here are some examples of TZ values that directly specify the time zone
       rules; they use some of the extensions to POSIX.

       EST5   stands for US Eastern Standard Time (EST), 5 hours behind UTC,
              without daylight saving.

       FJT-12FJST,M10.3.1/146,M1.3.4/75
              stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours
              ahead of UTC, springing forward on October's third Monday at
              146:00 (i.e., 02:00 on the first Sunday on or after October 21),
              and falling back on January's third Thursday at 75:00 (i.e.,
              03:00 on the first Sunday on or after January 18).

       IST-2IDT,M3.4.4/26,M10.5.0
              stands for Israel Standard Time (IST) and Israel Daylight Time
              (IDT), 2 hours ahead of UTC, springing forward on March's fourth
              Thursday at 26:00 (i.e., 02:00 on the first Friday on or after
              March 23), and falling back on October's last Sunday at 02:00.

       WART4WARST,J1/0,J365/25
              stands for Western Argentina Summer Time (WARST), 3 hours behind
              UTC.  There is a dummy fall-back transition on December 31 at
              25:00 daylight saving time (i.e., 24:00 standard time,
              equivalent to January 1 at 00:00 standard time), and a
              simultaneous spring-forward transition on January 1 at 00:00
              standard time, so daylight saving time is in effect all year and
              the initial WART is a placeholder.

       WGT3WGST,M3.5.0/-2,M10.5.0/-1
              stands for Western Greenland Time (WGT) and Western Greenland
              Summer Time (WGST), 3 hours behind UTC, where clocks follow the
              EU rules of springing forward on March's last Sunday at 01:00
              UTC (-02:00 local time) and falling back on October's last
              Sunday at 01:00 UTC (-01:00 local time).

       If no rule is present in TZ, the rules specified by the
       tzfile(5)-format file posixrules in the system time conversion
       information directory are used, with the standard and summer time
       offsets from UTC replaced by those specified by the offset values in
       TZ.

       For compatibility with System V Release 3.1, a semicolon (;) may be
       used to separate the rule from the rest of the specification.

       If the TZ environment variable does not specify a tzfile(5)-format and
       cannot be interpreted as a direct specification, UTC is used.

FILES
       /usr/local/etc/zoneinfo             time zone information directory
       /usr/local/etc/zoneinfo/localtime   local time zone file
       /usr/local/etc/zoneinfo/posixrules  used with POSIX-style TZ's
       /usr/local/etc/zoneinfo/GMT         for UTC leap seconds

       If /usr/local/etc/zoneinfo/GMT is absent, UTC leap seconds are loaded
       from /usr/local/etc/zoneinfo/posixrules.

SEE ALSO
       getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)

                                                                   NEWTZSET(3)
