man : ctime(3)
CTIME(3) OpenBSD Programmer's Manual CTIME(3)
asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r,
localtime, localtime_r, mktime, timegm, timelocal - convert date and time
extern char *tzname;
ctime(const time_t *clock);
ctime_r(const time_t *clock, char *buf);
difftime(time_t time1, time_t time0);
asctime(const struct tm *tm);
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);
mktime(struct tm *tm);
timegm(struct tm *tm);
timelocal(struct tm *tm);
The ctime() function converts a time_t, pointed to by clock, representing
the time in seconds since 00:00:00 UTC, 1970-01-01, 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 ctime_r() 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 localtime() 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().
gmtime() converts to Coordinated Universal Time.
The localtime_r() and gmtime_r() 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.
asctime() 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.
asctime_r() uses the buffer pointed to by buf (which should contain at
least 26 bytes) and then returns buf.
mktime() 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 time() 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
timelocal() is a deprecated interface that is equivalent to calling
mktime() with a negative value for tm_isdst.
timegm() is a deprecated interface that 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.
difftime() 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 UTC in seconds */
char *tm_zone; /* abbreviation of timezone name */
The tm_zone and tm_gmtoff fields exist, and are filled in by mktime(),
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 UTC,
with positive values indicating east of the Prime Meridian.
/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
getenv(3), strftime(3), time(3), tzset(3), tzfile(5), zic(8)
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
Avoid using out-of-range values with mktime() when setting up lunch with
promptness sticklers in Riyadh.
OpenBSD 4.9 August 23, 2010 OpenBSD 4.9