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TZSET(3)		     Library Functions Manual			TZSET(3)

NAME
     tzset, tzsetwall -- initialize time conversion information

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     void
     tzset(void);

     void
     tzsetwall(void);

DESCRIPTION
     The  tzset()  function  initializes time conversion information used by the
     library routine localtime(3).  The environment variable  TZ  specifies  how
     this is done.

     If  TZ does not appear in the environment, the best available approximation
     to local wall  clock  time,  as  specified  by  the  tzfile(5)-format  file
     /etc/localtime is used.

     If  TZ  appears  in the environment but its value is a null string, Coordi-
     nated Universal Time (UTC) is used (without leap second correction).

     If TZ appears in the environment and its value begins with a  colon  (`:'),
     the rest of its value is used as a pathname of a tzfile(5)-format file from
     which  to	read the time conversion information.  If the first character of
     the pathname is a slash (`/') it is used as an  absolute  pathname;  other-
     wise,  it	is used as a pathname relative to the system time conversion in-
     formation directory.

     If its value does not begin with a colon, it is first used as the	pathname
     of  a  file (as described above) from which to read the time conversion in-
     formation.  If that file cannot be read, the value is then interpreted as a
     direct specification (the format is described below) of the time conversion
     information.

     If the TZ environment variable does not specify a tzfile(5)-format file and
     cannot be interpreted as a direct specification, UTC is used.

     The tzsetwall() function sets things up so that  localtime(3)  returns  the
     best available approximation of local wall clock time.

SPECIFICATION FORMAT
     When TZ is used directly as a specification of the time conversion informa-
     tion, it must have the following syntax (spaces inserted for clarity):

	   std offset [dst [offset] [, rule]]

     Where:

	   std and dst	Three  or  more  bytes	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  one must add 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  pre-
			ceded  by  a  (`-')  the  time zone shall be east of the
			Prime Meridian; otherwise it shall be west (which may be
			indicated by an optional preceding (`+')).

	   rule 	Indicates when to change to and back from  summer  time.
			The rule has the form:

			      date/time,date/time

			where  the  first  date  describes  when the change from
			standard to summer time occurs and the second  date  de-
			scribes  when  the change back happens.  Each time field
			describes when, in current local time, the change to the
			other time is made.

			The format of date is one of the following:

			J n	 The Julian day n (1 <= n <=  365).   Leap  days
				 are  not  counted; that is, in all years -- in-
				 cluding leap years -- February 28 is day 59 and
				 March 1 is day 60.  It is impossible to explic-
				 itly 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.

			M m.n.d  The d'th day (0 <= d <= 6) of week n of month m
				 of the year (1 <= n <= 5),  (1  <=  m	<=  12),
				 where	week 5 means "the last d day in month m"
				 which may occur in either  the  fourth  or  the
				 fifth week).  Week 1 is the first week in which
				 the d'th day occurs.  Day zero is Sunday.

				 The  time  has the same format as offset except
				 that no leading sign (`-') or (`+') is allowed.
				 The default, if time is not given, is 02:00:00.

			If no rule is present in the TZ specification, the rules
			specified by the tzfile(5)-format file posixrules in the
			system time conversion information directory  are  used,
			with  the  standard and summer time offsets from UTC re-
			placed by those specified by the offset values in TZ.

     For compatibility with System V Release 3.1, a semicolon (`;') may be  used
     to separate the rule from the rest of the specification.

FILES
     /etc/localtime		     local time zone file
     /usr/share/zoneinfo	     time zone directory
     /usr/share/zoneinfo/posixrules  rules for POSIX-style TZ's
     /usr/share/zoneinfo/Etc/GMT     for UTC leap seconds

     If  the  file  /usr/share/zoneinfo/UTC does not exist, UTC leap seconds are
     loaded from /usr/share/zoneinfo/posixrules.

SEE ALSO
     date(1), gettimeofday(2), ctime(3), getenv(3), time(3), tzfile(5)

HISTORY
     The tzset() and tzsetwall() functions first appeared in 4.4BSD.

FreeBSD 12.1			November 17, 1993			TZSET(3)
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