CTIME(3) FreeBSD Library Functions Manual CTIME(3)
NAME
asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r,
localtime, localtime_r, mktime, offtime, offtime_r, timegm - transform
binary date and time values
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <time.h>
extern char *tzname[2];
char *
asctime(const struct tm *tm);
char *
asctime_r(const struct tm *tm, char *buf);
char *
ctime(const time_t *clock);
char *
ctime_r(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 *clock, struct tm *result);
struct tm *
localtime(const time_t *clock);
struct tm *
localtime_r(const time_t *clock, struct tm *result);
time_t
mktime(struct tm *tm);
struct tm *
offtime(const time_t *clock, long offset);
struct tm *
offtime_r(const time_t *clock, long offset, struct tm *result);
time_t
timegm(struct tm *tm);
DESCRIPTION
The ctime(), gmtime(), localtime(), and offtime() functions all take as
argument a pointer to a time value representing the time in seconds since
the Epoch (00:00:00 UTC on January 1, 1970; see time(3)).
The localtime() function converts the time value pointed to by clock, and
returns a pointer to a struct tm (described below) which contains the
broken-out time information for the value after adjusting for the current
time zone (see tzset(3)). When the specified time translates to a year
that will not fit in an int, localtime() returns NULL. The localtime()
function uses tzset(3) to initialize time conversion information if
tzset(3) has not already been called by the process.
After filling in the struct tm, localtime() sets the tm_isdst'th element
of tzname to a pointer to an ASCII string that is the time zone
abbreviation to be used with localtime()'s return value.
The gmtime() function similarly converts the time value, but without any
time zone adjustment, and returns a pointer to a struct tm.
The offtime() function similarly converts the time value with a time zone
adjustment corresponding to the provided offset, which is expressed in
seconds, with positive values indicating a time zone ahead of UTC (east
of the Prime Meridian). It does not call tzset(3) or modify tzname.
The ctime() function adjusts the time value for the current time zone in
the same manner as localtime(), and returns a pointer to a 26-character
string of the form:
Thu Nov 24 18:22:48 1986\n\0
All the fields have constant width.
The asctime() function converts the broken down time in the struct tm
pointed to by tm to the form shown in the example above.
The ctime_r() and asctime_r() functions provide the same functionality as
ctime() and asctime() except the caller must provide the output buffer
buf, which must be at least 26 characters long, to store the result in.
The localtime_r(), gmtime_r(), and offtime_r() functions provide the same
functionality as localtime(), gmtime(), and offtime() respectively,
except the caller must provide the output buffer result.
The functions mktime() and timegm() convert the broken-down time in the
struct tm pointed to by tm into a time value with the same encoding as
that of the values returned by the time(3) function (that is, seconds
from the Epoch, UTC). The mktime() function interprets the input
structure according to the current timezone setting (see tzset(3)) while
the timegm() function interprets the input structure as representing
Universal Coordinated Time (UTC).
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
needed. For example, October 40 is changed into November 9, a tm_hour of
-1 means 1 hour before midnight, tm_mday of 0 means the day preceding the
current month, and tm_mon of -2 means 2 months before January of tm_year.
(A positive or zero value for tm_isdst causes mktime() to presume
initially that summer time (for example, Daylight Saving Time) is or is
not in effect for the specified time, respectively. A negative value for
tm_isdst causes the mktime() function to attempt to guess whether summer
time is in effect for the specified time. The tm_isdst and tm_gmtoff
members are forced to zero by timegm().)
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 mktime() function
returns the specified calendar time; if the calendar time cannot be
represented, it returns -1 and sets errno(3) to an appropriate value.
Note that -1 is a valid result (representing one second before midnight
UTC on the evening of 31 December 1969), so this cannot be relied upon to
indicate success or failure; instead, tm_wday and / or tm_yday should be
set to an out-of-bounds value (e.g. -1) prior to calling mktime() or
timegm() and checked after the call.
The difftime() function returns the difference in seconds between two
time values, time1 - time0.
External declarations as well as the definition of struct tm are in the
<time.h> header. The tm structure includes at least 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? */
char *tm_zone; /* abbreviation of timezone name */
long tm_gmtoff; /* offset from UTC in seconds */
The tm_isdst field is non-zero if summer time is in effect.
The tm_gmtoff field is the offset in seconds of the time represented from
UTC, with positive values indicating a time zone ahead of UTC (east of
the Prime Meridian).
SEE ALSO
date(1), clock_gettime(2), gettimeofday(2), getenv(3), time(3), tzset(3),
tzfile(5)
STANDARDS
The asctime(), ctime(), difftime(), gmtime(), localtime(), and mktime()
functions conform to ISO/IEC 9899:1990 ("ISO C90"), and conform to
ISO/IEC 9945-1:1996 ("POSIX.1") provided the selected local timezone does
not contain a leap-second table (see zic(8)).
The asctime_r(), ctime_r(), gmtime_r(), and localtime_r() functions are
expected to conform to ISO/IEC 9945-1:1996 ("POSIX.1") (again provided
the selected local timezone does not contain a leap-second table).
The timegm() function is expected to conform to ISO/IEC 9899:2024
("ISO C23") with the same constraint.
HISTORY
This manual page is derived from the time package contributed to Berkeley
by Arthur Olson and which appeared in 4.3BSD.
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 asctime_r(), ctime_r(), gmtime_r() and localtime_r() functions have
been available since FreeBSD 8.0. The offtime() and offtime_r()
functions were added in FreeBSD 15.0.
BUGS
Except for difftime(), mktime(), and the _r() variants of the other
functions, these functions leave their result in an internal static
object and return a pointer to that object. Subsequent calls to these
function will modify the same object.
The C Standard provides no mechanism for a program to modify its current
local timezone setting, and the POSIX-standard method is not reentrant.
(However, thread-safe implementations are provided in the POSIX threaded
environment.)
The tm_zone field of a returned tm structure points to a static array of
characters, which will also be overwritten by any subsequent calls (as
well as by subsequent calls to tzset(3)).
Use of the external variable tzname is discouraged; the tm_zone entry in
the tm structure is preferred.
FreeBSD 15.1-STABLE-HBSD May 14, 2026 CTIME(3)