NAME

asctime, asctime_r, ctime, ctime_r, ctime_rz, difftime, gmtime, gmtime_r, localtime, localtime_r, localtime_rz, mktime, mktime_z — convert date and time

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <time.h>

extern char *tzname[2];

[[deprecated]] char *
asctime(const struct tm *tm);

[[only in POSIX.1-2017 and earlier]] char *
asctime_r(const struct tm *restrict tm, char * restrict buf);

[[deprecated]] char *
ctime(const time_t *clock);

[[only in POSIX.1-2017 and earlier]] char *
ctime_r(const time_t *clock, char *buf);

char *
ctime_rz(timezone_t restrict tz, const time_t *clock, char *buf);

double
difftime(time_t time1, time_t time0);

struct tm *
gmtime(const time_t *clock);

struct tm *
gmtime_r(const time_t * restrict clock, struct tm * restrict result);

struct tm *
localtime(const time_t *clock);

struct tm *
localtime_r(const time_t * restrict clock, struct tm * restrict result);

struct tm *
localtime_rz(timezone_t restrict tz, const time_t * restrict clock, struct tm * restrict result);

time_t
mktime(struct tm *tm);

time_t
mktime_z(timezone_t restrict tz, struct tm *restrict tm);

DESCRIPTION

The asctime family of functions provide various standard library routines to operate with time and conversions related to time.

FUNCTIONS

asctime(tm)

The asctime() function converts a time value contained in the tm structure to a string with the following general format:

Thu Nov 24 18:22:48 1986\n\0

Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form:

Thu Nov 24 18:22:48     8198600

with five spaces before the year. This unusual format is designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years. This function is deprecated starting in C23. Callers can use strftime() instead.

The tm structure is described in tm(3).

This function is deprecated starting in C23. Callers can use strftime() instead.

asctime_r(tm, buf)

The asctime_r() has the same behavior as asctime(), but the result is stored in buf, which should have a size of at least 26 bytes.

ctime(clock)

The ctime() function converts a time_t, pointed to by clock, and returns a pointer to a string with the format described above. Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form

Thu Nov 24 18:22:48     81986\n\0

with five spaces before the year. These unusual formats are designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years.

The clock time stamp represents the time in seconds since 1970-01-01 00:00:00 Coordinated Universal Time (UTC). The POSIX standard says that time stamps must be nonnegative and must ignore leap seconds. Many implementations extend POSIX by allowing negative time stamps, and can therefore represent time stamps that predate the introduction of UTC and are some other flavor of Universal Time (UT). Some implementations support leap seconds, in contradiction to POSIX.

The ctime() function is deprecated starting in C23. Callers can use localtime_r() and strftime() instead.

ctime_r(clock, buf)

The ctime_r() is similar to ctime(), except it places the result of the conversion in the buf argument, which should be 26 or more bytes long, instead of using a global static buffer.

ctime_rz(tz, clock, buf)

The ctime_rz() function is similar to ctime_r(), but it also takes a timezone_t argument, as returned by a previous call to tzalloc(), or a NULL pointer denoting Coordinated Universal Time (UTC).

difftime(time1, time2)

The difftime() function returns the difference between two calendar times, (time1 - time0), expressed in seconds.

gmtime(clock)

The gmtime() function converts to Coordinated Universal Time (UTC) and returns a pointer to the tm structure described in tm(3).

gmtime_r(clock, result)

The gmtime_r() function provides the same functionality as gmtime(), differing in that the caller must supply a buffer area result in which the result is stored.

localtime(clock)

Also localtime() is comparable to gmtime(). However, localtime() corrects for the timezone and any timezone adjustments (such as Daylight Saving Time in the U.S.A.). After filling in the tm structure, the function sets the tm_isdst'th element of tzname to a pointer to an ASCII string that is the timezone abbreviation to be used with localtime()'s return value.

localtime_r(clock, result)

As gmtime_r(), the localtime_r() takes an additional buffer result as a parameter and stores the result in it. Note however that localtime_r() does not imply initialization of the local time conversion information; the application may need to do so by calling tzset(3).

localtime_rz(tz, clock, result)

The localtime_rz() function is similar to localtime_r(), but it also takes a timezone_t argument, returned by a previous call to tzalloc(), or a NULL pointer denoting Coordinated Universal Time (UTC).

mktime(tm)

The mktime() function converts the broken-down time, expressed as local time in the tm(3) structure, into a calendar time value with the same encoding as that of the values returned by the time(3) function. The following remarks should be taken into account.

The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to their normal ranges and will be normalized, if need be.
For example, consider a struct tm initialized with tm_year = 122, tm_mon = 10, tm_mday = 30, tm_hour = 22, tm_min = 57, and tm_sec = 0. Incrementing tm_min by 13 and calling mktime() would lead to tm_hour = 23 and tm_min = 10.

This normalizing can lead to cascading changes: Again using a struct tm initialized as in the above example but with tm_hour = 23, the same change would lead to tm_mon = 11, tm_mday = 1, tm_hour = 0, and tm_min = 10.

Negative values may also be normalized with similar cascading effect such that e.g., a tm_hour of -1 means 1 hour before midnight on the previous day and so on.
A positive or zero value for tm_isdst causes mktime() to presume initially that daylight saving time respectively, is or is not in effect for the specified time.
A negative value for tm_isdst causes the mktime() function to attempt to divine whether daylight saving time is in effect for the specified time; in this case it does not use a consistent rule and may give a different answer when later presented with the same argument.
On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to their normal ranges; the final value of tm_mday is not set until tm_mon and tm_year are determined.

The function returns the specified calendar time; if the calendar time cannot be represented, it returns (time_t)-1. This can happen either because the resulting conversion would not fit in a time_t variable, or because the time specified happens to be in the daylight savings gap and tm_isdst was set to -1. Other mktime() implementations do not return an error in the second case and return the appropriate time offset after the daylight savings gap. There is code to mimick this behavior, but it is not enabled by default.

mktime_z(tz, tm)

The mktime_z() function is similar to mktime() but it also takes a const timezone_t argument, returned by a previous call to tzalloc(), or a null pointer denoting Coordinated Universal Time (UTC).

Declarations of all the functions and externals, and the tm structure, are in the <time.h> header file. The structure (of type) struct tm includes the following fields:

       int tm_sec;      /* seconds (0 - 60) */
       int tm_min;      /* minutes (0 - 59) */
       int tm_hour;     /* hours (0 - 23) */
       int tm_mday;     /* day of month (1 - 31) */
       int tm_mon;      /* month of year (0 - 11) */
       int tm_year;     /* year - 1900 */
       int tm_wday;     /* day of week (Sunday = 0) */
       int tm_yday;     /* day of year (0 - 365) */
       int tm_isdst;    /* is daylight saving time in effect? */
       char *tm_zone;   /* abbreviation of timezone name (optional) */
       long tm_gmtoff;  /* offset from UT in seconds (optional) */

tm_isdst is non-zero if daylight saving time is in effect.
tm_gmtoff is the offset (in seconds) of the time represented from UT, with positive values indicating east of the Prime Meridian. The field's name is derived from Greenwich Mean Time, a precursor of UT.

In struct tm the tm_zone and tm_gmtoff fields exist, and are filled in, only if arrangements to do so were made when the library containing these functions was created. Similarly, the tzname variable is optional; also, there is no guarantee that tzname will continue to exist in this form in future releases of this code.

The ctime_r(), localtime_r(), gmtime_r(), and asctime_r() functions are like their unsuffixed counterparts, except that they accept an additional argument specifying where to store the result if successful.

The ctime_rz(), localtime_rz(), and mktime_z() functions are like their unsuffixed counterparts, except that they accept an extra initial zone argument specifying the timezone to be used for conversion. If zone is NULL, UT is used; otherwise, zone should have been allocated by tzalloc() and should not be freed until after all uses (e.g., by calls to strftime()) of the filled-in tm_zone() fields.

RETURN VALUES

On success the asctime() and ctime() functions return a pointer to a static character buffer, and the asctime_r(), ctime_r(), and ctime_rz() function return a pointer to the user-supplied buffer. On failure they all return NULL and no errors are defined for them.
On success the gmtime(), and localtime() functions return a pointer to a statically allocated struct tm whereas the gmtime_r(), localtime_r(), and localtime_rz(), functions return a pointer to the user-supplied struct tm. On failure they all return NULL and the global variable errno is set to indicate the error.
The mktime() and mktime_z() function returns the specified time since the Epoch as a time_t type value. If the time cannot be represented, then mktime() and mktime_z() return (time_t)-1 setting the global variable errno to indicate the error.
The tzalloc() function returns a pointer to a timezone_t object or NULL on failure, setting errno to indicate the error. It may also return NULL when the name argument is NULL, and this is not an error, but a way of referring to Coordinated Universal Time (UTC).
tzgetzone() function returns string containing the name of the timezone given in tz.

FILES

/etc/localtime: local timezone file
/usr/share/zoneinfo: timezone information directory
/usr/share/zoneinfo/localtime: local timezone file
/usr/share/zoneinfo/posixrules: default DST rules (obsolete, and can cause bugs if present)
/usr/share/zoneinfo/GMT: for UTC leap seconds

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

ERRORS

The described functions may fail with

[EINVAL]: The result cannot be represented because a parameter is incorrect, or the conversion failed because no such time exists (for example a time in the DST gap).
[EOVERFLOW]: The result cannot be represented because the time requested is out of bounds and the time calculation resulted in overflow.

All functions that return values, except their “z” variants, can also return the same errors as open(2) and malloc(3).

STANDARDS

The ctime(), difftime(), asctime(), localtime(), gmtime() and mktime() functions conform to ANSI X3.159-1989 (“ANSI C89”). Rest of the functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

HISTORY

A ctime() function appeared in Version 1 AT&T UNIX.

CAVEATS

The functions that do not take an explicit timezone_t argument return values pointing to static data; the data is overwritten by each call. For the above functions the tzname variable (once set) and the tm_zone field of a returned struct tm point to an array of characters that can be freed or overwritten by later calls to the functions localtime(), tzfree(), and tzset(), if these functions affect the timezone information that specifies the abbreviation in question. The remaining functions and data are thread-safe. The functions that do take an explicit timezone_t argument and set the fields of a supplied struct tm should not call tzfree() since the tm_zone field of the struct tm points to data allocated by tzalloc().

The asctime(), asctime_r(), ctime(), ctime_r(), and ctime_rz(), functions behave strangely for years before 1000 or after 9999. The 1989 and 1999 editions of the C Standard say that years from -99 through 999 are converted without extra spaces, but this conflicts with longstanding tradition and with this implementation. The 2011 edition says that the behavior is undefined if the year is before 1000 or after 9999. Traditional implementations of these two functions are restricted to years in the range 1900 through 2099. To avoid this portability mess, new programs should use strftime() instead.