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CTIME(3) Library Functions Manual CTIME(3)

asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r, localtime, localtime_r, mktime, timegm, timelocal — convert date and time to ASCII

#include <sys/types.h>
#include <time.h>

extern char *tzname[2];

char *
ctime(const time_t *clock);

char *
ctime_r(const time_t *clock, char *buf);

double
difftime(time_t time1, time_t time0);

char *
asctime(const struct tm *tm);

char *
asctime_r(const struct tm *tm, char *buf);

struct tm *
localtime(const time_t *clock);

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

struct tm *
gmtime(const time_t *clock);

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

time_t
mktime(struct tm *tm);

time_t
timegm(struct tm *tm);

time_t
timelocal(struct tm *tm);

The () function converts a time_t, pointed to by clock, and returns a pointer to a string of the form

Thu Nov 24 18:22:48 1986\n

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

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 () function converts the calendar time pointed to by clock to local time in exactly the same way as ctime() and puts the string into the array pointed to by buf (which contains at least 26 bytes) and returns buf. Unlike ctime(), the thread-safe version ctime_r() is not required to set tzname.

The () and gmtime() functions return pointers to tm structures, described below. localtime() corrects for the time zone and any time zone adjustments (such as Daylight Saving Time in the United States). After filling in the tm structure, localtime() sets the tm_isdst'th element of tzname to a pointer to an ASCII string that's the time zone abbreviation to be used with the return value of localtime().

() converts to Coordinated Universal Time.

The () and () functions convert the calendar time pointed to by clock into a broken-down time in exactly the same way as their non-reentrant counterparts, localtime() and gmtime(), but instead store the result directly into the structure pointed to by result. Unlike localtime(), the reentrant version is not required to set tzname.

() converts a time value contained in a tm structure to a string, as shown in the above example, and returns a pointer to the string. () uses the buffer pointed to by buf (which should contain at least 26 bytes) and then returns buf.

() converts the broken-down time, expressed as local time, in the structure pointed to by tm into a calendar time value with the same encoding as that of the values returned by the () function. 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. (A positive or zero value for tm_isdst causes mktime() to presume initially that summer time (for example, Daylight Saving Time in the U.S.A.) 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 summer 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. mktime() returns the specified calendar time; if the calendar time cannot be represented, it returns -1.

() is a deprecated interface that is equivalent to calling mktime() with a negative value for tm_isdst.

() converts the broken-down time, as returned by gmtime(), into a calendar time value with the same encoding as that of the values returned by the time() function.

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

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 summer time in effect? */
	long tm_gmtoff;	/* offset from UT in seconds */
	char *tm_zone;	/* abbreviation of timezone name */

The tm_zone and tm_gmtoff fields exist, and are filled in by (), localtime(), timegm(), and gmtime(), but are not standardized. There is no guarantee that these fields will continue to exist in this form and they may be altered or removed in a future release.

tm_isdst is non-zero if summer 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.

The functions ctime(), ctime_r(), asctime(), asctime_r(), localtime(), localtime_r(), gmtime() and gmtime_r() return NULL on error. The function mktime() returns -1 on error.

/usr/share/zoneinfo
time zone information directory
/etc/localtime
local time zone file
/usr/share/zoneinfo/posixrules
used with POSIX-style TZ's
/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.

getenv(3), strftime(3), time(3), tzset(3), tzfile(5), zic(8)

The functions asctime(), ctime(), difftime(), gmtime(), localtime(), and mktime() conform to ANSI X3.159-1989 (“ANSI C89”).

The functions asctime_r(), ctime_r(), gmtime_r(), and localtime_r() conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The functions timegm() and timelocal() are extensions to these standards.

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

The functions asctime(), gmtime(), and localtime() first appeared in Version 5 AT&T UNIX, difftime() and mktime() in 4.3BSD-Reno, and timegm() and timelocal() in SunOS 4.0.

The functions asctime_r(), ctime_r(), gmtime_r(), and localtime_r() have been available since OpenBSD 2.5.

The return values of the non re-entrant functions point to static data; the data is overwritten by each call. The tm_zone field of a returned struct tm points to a static array of characters, which will also be overwritten at the next call (and by calls to tzset(3)).

asctime() and ctime() 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. 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.

The default system time zone may be set by running “zic -l timezone” as the superuser.

Avoid using out-of-range values with mktime() when setting up lunch with promptness sticklers in Riyadh.

June 23, 2025 OpenBSD-current