asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r,
localtime, localtime_r, mktime, timegm -- transform binary date and time
Standard C Library (libc, -lc)
extern char *tzname;
ctime(const time_t *clock);
difftime(time_t time1, time_t time0);
asctime(const struct tm *tm);
struct tm *
localtime(const time_t *clock);
struct tm *
gmtime(const time_t *clock);
mktime(struct tm *tm);
timegm(struct tm *tm);
ctime_r(const time_t *clock, char *buf);
struct tm *
localtime_r(const time_t *clock, struct tm *result);
struct tm *
gmtime_r(const time_t *clock, struct tm *result);
asctime_r(const struct tm *tm, char *buf);
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)).
The function localtime() converts the time value pointed at 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 (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 that's the time zone
abbreviation to be used with localtime()'s return value.
The function gmtime() similarly converts the time value, but without any
time zone adjustment, and 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(), 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 ctime_r() function provides the same functionality as ctime() except
the caller must provide the output buffer buf to store the result, which
must be at least 26 characters long. 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 down time in the structure tm
pointed at by *tm to the form shown in the example above.
The asctime_r() function provides the same functionality as asctime()
except the caller provide the output buffer buf to store the result,
which must be at least 26 characters long.
The functions mktime() and timegm() convert the broken-down time in the
structure 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)). 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 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 represented,
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 in the
<time.h> include file. The tm structure includes at least the following
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 time is in effect.
The field tm_gmtoff is the offset (in seconds) of the time represented
from UTC, with positive values indicating east of the Prime Meridian.
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 C89''), 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 not specified by any standard; its function cannot
be completely emulated using the standard functions described above.
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 reentrant.
(However, thread-safe implementations are provided in the POSIX threaded
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.
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