BSD 2.11 - man page for localtime (bsd section 3)

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CTIME(3)										 CTIME(3)

       ctime, localtime, gmtime, asctime, timezone, tzset -  convert date and time to ASCII

       void tzset()

       char *ctime(clock)
       time_t *clock;

       #include <time.h>

       char *asctime(tm)
       struct tm *tm;

       struct tm *localtime(clock)
       time_t *clock;

       struct tm *gmtime(clock)
       time_t *clock;

       char *timezone(zone, dst)

       Tzset uses the value of the environment variable TZ to set up the time conversion informa-
       tion used by localtime.

       If TZ does not appear in the environment, the TZDEFAULT file (as defined in  tzfile.h)  is
       used  by  localtime.  If this file fails for any reason, the GMT offset as provided by the
       kernel is used.	In this case, DST is ignored, resulting in the time  being  incorrect  by
       some amount if DST is currently in effect.  If this fails for any reason, GMT is used.

       If  TZ  appears	in the environment but its value is a null string, Greenwich Mean Time is
       used; if TZ appears and begins with a slash, it is used as the absolute	pathname  of  the
       tzfile(5)-format  file  from  which to read the time conversion information; if TZ appears
       and begins with a character other than a slash, it's used as a pathname	relative  to  the
       system  time  conversion  information  directory,  defined  as  TZDIR  in the include file
       tzfile.h.  If this file fails for any reason, GMT is used.

       Programs that always wish to use local wall clock time should explicitly remove the  envi-
       ronmental variable TZ with unsetenv(3).

       Ctime converts a long integer, pointed to by clock, such as returned by time(2) into ASCII
       and returns a pointer to a 26-character string in the following form.  All the fields have
       constant width.

	   Sun Sep 16 01:03:52 1973\n

       Localtime  and  gmtime  return  pointers  to  structures  containing the broken-down time.
       Localtime corrects for the time zone and possible daylight savings time;  gmtime  converts
       directly  to  GMT,  which  is  the time UNIX uses.  Asctime converts a broken-down time to
       ASCII and returns a pointer to a 26-character string.

       The structure declaration from the include file is:

	      struct tm {
		   int tm_sec;	  /* 0-59  seconds */
		   int tm_min;	  /* 0-59  minutes */
		   int tm_hour;   /* 0-23  hour */
		   int tm_mday;   /* 1-31  day of month */
		   int tm_mon;	  /* 0-11  month */
		   int tm_year;   /* 0-    year - 1900 */
		   int tm_wday;   /* 0-6   day of week (Sunday = 0) */
		   int tm_yday;   /* 0-365 day of year */
		   int tm_isdst;  /* flag: daylight savings time in effect */
		   char **tm_zone;	   /* abbreviation of timezone name */
		   long tm_gmtoff;	   /* offset from GMT in seconds */

       Tm_isdst is non-zero if a time zone adjustment such as Daylight Savings time is in effect.

       Tm_gmtoff is the offset (in seconds) of the time represented from GMT, with positive  val-
       ues indicating East of Greenwich.

       Timezone  remains  for  compatibility  reasons only; it's impossible to reliably map time-
       zone's arguments (zone, a "minutes west of GMT" value and dst, a "daylight saving time  in
       effect" flag) to a time zone abbreviation.

       If  the environmental string TZNAME exists, timezone returns its value, unless it consists
       of two comma separated strings, in which case the second string is returned if dst is non-
       zero, else the first string.  If TZNAME doesn't exist, zone is checked for equality with a
       built-in table of values, in which case timezone returns the time zone  or  daylight  time
       zone  abbreviation  associated  with that value.  If the requested zone does not appear in
       the table, the difference from GMT is returned; e.g. in Afghanistan,  timezone(-(60*4+30),
       0)  is  appropriate  because it is 4:30 ahead of GMT, and the string GMT+4:30 is returned.
       Programs that in the past used the timezone function should return the zone name as set by
       localtime to assure correctness.

       /usr/share/zoneinfotime zone information directory
       /etc/localtime	  local time zone file

       gettimeofday(2), getenv(3), time(3), tzfile(5), environ(7)

       The  return  values  point  to static data whose content 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).

4th Berkeley Distribution		November 27, 1996				 CTIME(3)
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