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ctime(3)							      ctime(3)

Name
       ctime,  localtime, gmtime, asctime, difftime, mktime, timezone, tzset -
       date and time functions

Syntax
       As shown, the and calls are common to both the non-System V environment
       and the System V environment.

   Common to Both Environments
       #include <time.h>

       void tzset()

       char *ctime(clock)
       time_t *clock;

       char *asctime(tm)
       struct tm *tm;

       struct tm *localtime(clock)
       time_t *clock;

       struct tm *gmtime(clock)
       time_t *clock;

       double difftime(time1, time0)
       time_t time1, time0;

       time_t mktime(timeptr)
       struct tm *timeptr;

       extern char *tzname[2];

   BSD Environment Only
       char *timezone(zone, dst)

   System V and POSIX Environments Only
       extern long timezone;

       extern int daylight;

Description
       The  call  uses	the value of the environment variable TZ to set up the
       time conversion information used by

       If TZ does not appear in the environment, the file is used by  If  this
       file fails for any reason, the Greenwich Mean Time (GMT) offset as pro‐
       vided by the kernel is used.  In this case, Daylight Savings Time (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, GMT is
       used;  if  TZ  appears and its value is not a null string, its value is
       interpreted using rules specific to the System V and non-System V envi‐
       ronments.

       Programs	 that  always wish to use local wall clock time should explic‐
       itly remove the environmental variable TZ with

       The call converts a long integer, pointed to by clock, representing the
       time  in	 seconds  since	 00:00:00  GMT, January 1, 1970, 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 1985\n\0

       The  and	 calls return pointers to tm structures, described below.  The
       call corrects for the time zone and possible DST; converts directly  to
       GMT, which is the time the ULTRIX system uses.

       The  call converts a tm structure to a 26-character string, as shown in
       the previous example, and returns a pointer to the string.

       Declarations of all the functions and externals, and the tm  structure,
       are in the <time.h> header file.	 The structure declaration is:
       struct tm {
	       int tm_sec;     /∗ seconds (0 - 59) ∗/
	       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;   /∗ flag: daylight savings time in
				     effect ∗/
	       long tm_gmtoff; /∗ offset from GMT in seconds ∗/
	       char *tm_zone;  /∗ abbreviation of timezone name ∗/

       };
       tm_isdst is nonzero if DST is in effect.

       tm_gmtoff  is the offset (in seconds) of the time represented from GMT,
       with positive values indicating East of Greenwich.

       The call computes the difference between two calendar  times:  time1  -
       time0 and returns the difference expressed in seconds.

       The  call  converts  the	 broken-down  local  time  in the tm structure
       pointed to by timeptr into a calendar time value with the same encoding
       as  that of the values returned by The values of tm_wday and tm_yday in
       the structure are ignored, and the other values are not	restricted  to
       the  ranges  indicated  above  for the tm structure. A positive or zero
       value for tm_isdst causes to presume that DST, respectively, is	or  is
       not  in	effect	for  the  specified  time.  A negative value causes to
       attempt to determine whether DST is in effect for the  specified	 time.
       On  successful  completion,  the	 values of tm_wday and tm_yday are set
       appropriately, and the other components are set to represent the speci‐
       fied  calendar  time,  but with their values forced to the ranges indi‐
       cated above. If the calendar time cannot be represented,	 the  function
       returns the value (time_t)-1.

       The external variable tzname, contains the current time zone names. The
       function sets this variable.

   BSD and POSIX Environment Only
       If TZ appears in the environment and its value is not  a	 null  string,
       its value has one of three formats:

	   :

       or

	   :pathname

       or

	   stdoffset[dst[offset][,start[/time],end[/time]]]

       If TZ is the single colon format (first format), GMT is used.

       If  TZ is the colon followed by a pathname format (second), the charac‐
       ters following the colon specify a pathname of a format file from which
       to  read the time conversion information. If the pathname begins with a
       slash, it represents an absolute pathname; otherwise  the  pathname  is
       relative	 to  the  system time conversion information directory If this
       file fails for any reason, the GMT offset as provided by the kernel  is
       used.

       If  the first character in TZ is not a colon (third format), the compo‐
       nents of the string have the following meaning:

       std and dst    Three or more characters that are	 the  designation  for
		      the  standard  (std) or summer (dst) time zone. Only std
		      is required; if dst is missing, then  summer  time  does
		      not  apply  in this locale. Upper- and lowercase letters
		      are explicitly allowed. Any characters except a  leading
		      colon  (:),  digits, comma (,), minus (-), plus (+), and
		      ASCII NUL are allowed.

       offset	      Indicates the value to be added to  the  local  time  to
		      arrive at Coordinated Universal Time. The offset has the
		      form:

			  hh[:mm[:ss]]

		      The minutes (mm) and seconds (ss) are optional. The hour
		      (hh)  is	required and may be a single digit. The offset
		      following std is required. If  no	 offset	 follows  dst,
		      summer  time is assumed to be one hour ahead of standard
		      time. One or more digits	may  be	 used;	the  value  is
		      always  interpreted  as a decimal number.	 The hour must
		      be between zero and 24, and the minutes (and seconds)  -
		      if  present - between zero and 59. If preceded by a "-",
		      the time zone is east of the Prime  Meridian;  otherwise
		      it  is  west (which may be indicated by an optional pre‐
		      ceding "+").

       start and end  Indicates when to change to and back from	 summer	 time.
		      Start  describes	the date when the change from standard
		      to summer time occurs and end describes  the  date  when
		      the  change  back	 happens.  The format of start and end
		      must be one of the following:

		      Jn      The Julian day n (1 ≤ n ≤ 365).  Leap  days  are
			      not  counted.   That is, in all years, including
			      leap years, February 28 is day 59 and March 1 is
			      day  60. It is impossible to explicitly refer to
			      the occasional February 29.

		      n	      The zero-based Julian day (0 ≤ n	≤  365).  Leap
			      days are counted, and it is possible to refer to
			      February 29.

		      Mm.n.d  The nth d day of month m (1 ≤ n ≤ 5, 0 ≤ d ≤  6,
			      1 ≤ m ≤ 12). When n is 5 it refers to the last d
			      day of month m. Day 0 is Sunday.

       time	      The time field describes the time when, in current time,
		      the  change  to or from summer time occurs. Time has the
		      same format as offset except that	 no  leading  sign  (a
		      minus  sign  (-)	or  a  plus  sign (+)) is allowed. The
		      default, if time is not given, is 02:00:00.

       As an example of the previous format, if the  TZ	 environment  variable
       had the value EST5EDT4,M4.1.0,M10.5.0 it would describe the rule, which
       went into effect in 1987, for the Eastern time zone in the USA. Specif‐
       ically,	EST  would  be	the  designation for standard time, which is 5
       hours behind GMT. EDT would be the designation  for  DST,  which	 is  4
       hours  behind  GMT. DST starts on the first Sunday in April and ends on
       the last Sunday in October.  In both cases,  since  the	time  was  not
       specified,  the	change to and from DST would occur at the default time
       of 2:00 AM.

       The call remains for compatibility reasons only; it  is	impossible  to
       reliably	 map timezone's arguments (zone, a `minutes west of GMT' value
       and dst, a `daylight saving time in effect' flag) to a time zone abbre‐
       viation.

       If the environmental string TZNAME exists, 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 does not exist, zone is checked for equality with a built-in ta‐
       ble  of	values,	 in  which case 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; that
       is, in Afghanistan, 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
       function should return the zone name as set by to assure correctness.

   System V Environment Only
       If TZ appears in the environment its value specifies a  pathname	 of  a
       format  file from which to read the time conversion information. If the
       pathname begins with a slash, it represents an absolute pathname;  oth‐
       erwise  the pathname is relative to the system time conversion informa‐
       tion directory

       If TZ appears in the environment and using the value as a pathname of a
       format  file  fails for any reason, the value is assumed to be a three-
       letter time zone name followed by a number representing the  difference
       between local time and GMT in hours, followed by an optional three-let‐
       ter name for a time zone on DST.	 For example, the setting for New Jer‐
       sey would be EST5EDT.

   System V and POSIX Environment Only
       The external long variable contains the difference, in seconds, between
       GMT and local standard time (in EST, is 5*60*60).  The  external	 vari‐
       able  daylight  is nonzero if any daylight time rules are to be applied
       (that is, if the specified timezone includes rules for daylight	time).
       These variables are set whenever or are called.

Restrictions
       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

Files
       /etc/zoneinfo		time zone information directory
       /etc/zoneinfo/localtime	local time zone file

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

								      ctime(3)
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