CTIME(3) BSD Library Functions Manual CTIME(3)
asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r, localtime, localtime_r, mktime, timegm -- transform binary date and time val-
Standard C Library (libc, -lc)
extern char *tzname;
asctime(const struct tm *timeptr);
asctime_r(const struct tm *restrict timeptr, char *restrict buf);
ctime(const time_t *clock);
ctime_r(const time_t *clock, char *buf);
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);
mktime(struct tm *timeptr);
timegm(struct tm *timeptr);
The functions ctime(), gmtime(), and localtime() all take as an argument a time value representing the time in seconds since the Epoch
(00:00:00 UTC, January 1, 1970; see time(3)). When encountering an error, these functions return NULL and set errno to an appropriate value.
The function localtime() converts the time value pointed at by clock. It returns a pointer to a ``struct tm'' (described below), which con-
tains the broken-out time information for the value after adjusting for the current time zone (and any other factors such as Daylight Saving
Time). Time zone adjustments are performed as specified by the TZ environment variable (see tzset(3)). The function localtime() uses
tzset(3) to initialize time conversion information, if tzset(3) has not already been called by the process.
After filling in the tm structure, localtime() sets the tm_isdst'th element of tzname to a pointer to an ASCII string containing the time
zone abbreviation to be used with localtime()'s return value.
The function gmtime() also converts the time value, but makes no time zone adjustment. It returns a pointer to a tm structure (described
The ctime() function adjusts the time value for the current time zone, in the same manner as localtime(). It returns a pointer to a 26-char-
acter string of the form:
Thu Nov 24 18:22:48 1986
All of the fields have constant width.
The ctime_r() function provides the same functionality as ctime(), except that the caller must provide the output buffer buf (which must be
at least 26 characters long) to store the result. The localtime_r() and gmtime_r() functions provide the same functionality as localtime()
and gmtime(), respectively, except the caller must provide the output buffer result.
The asctime() function converts the broken-out time in the structure tm (pointed at by *timeptr) to the form shown in the example above.
The asctime_r() function provides the same functionality as asctime(), except that the caller provides the output buffer buf (which must be
at least 26 characters long) to store the result.
The functions mktime() and timegm() convert the broken-out time (in the structure pointed to by *timeptr) 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)). The timegm() function interprets the input structure as represent-
ing Universal Coordinated Time (UTC).
The original values of the tm_wday and tm_yday components of the structure are ignored. The original values of the other components are not
restricted to their normal ranges and will be normalized, if need be. 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 (respectively) 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. 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 repre-
sented, it returns -1;
The difftime() function returns the difference between two calendar times, (time1 - time0), expressed in seconds.
External declarations, as well as the tm structure definition, are contained in the <time.h> include file. 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 field tm_isdst is non-zero if summer (i.e., Daylight Saving) time is in effect.
The field tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values indicating locations east of the Prime
date(1), gettimeofday(2), getenv(3), time(3), tzset(3), tzfile(5)
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 pro-
vided the selected local timezone does not contain a leap-second table).
The timegm() function is not specified by any standard; its function cannot be completely emulated using the standard functions described
This manual page is derived from the time package contributed to Berkeley by Arthur Olson and which appeared in 4.3BSD.
Except for difftime(), mktime(), and the _r() variants of the other functions, these functions leaves 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 reen-
trant. (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) and tzsetwall(3)).
Use of the external variable tzname is discouraged; the tm_zone entry in the tm structure is preferred.
January 2, 1999 BSD