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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	speci-
       fies how	this is	done.

       If TZ does not appear in	the environment, the best available approxima-
       tion  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,	 Coor-
       dinated 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 informa-
       tion.  If the first character of	the pathname is	a slash	 (`/')	it  is
       used as an absolute pathname; otherwise,	it is used as a	pathname rela-
       tive to the system time conversion information directory.

       If its value does not begin with	a colon, it is first used as the path-
       name of a file (as described above) from	which to read the time conver-
       sion  information.   If that file cannot	be read, the value is then in-
       terpreted 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  in-
       formation, it must have the following syntax (spaces inserted for clar-
       ity):

	     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  ex-
			  cept a leading colon (`:'), digits, comma (`,'), mi-
			  nus (`-'), 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 preceded 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
			  describes 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
				   --  including  leap years --	February 28 is
				   day 59 and March 1 is day 60.  It is	impos-
				   sible to explicitly refer to	the occasional
				   February 29.

			  n	   The zero-based Julian day (0	<= n <=	365  )
				   .   Leap days are counted, and it is	possi-
				   ble 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  ex-
				   cept	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	replaced 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	13.2		       November	17, 1993		      TZSET(3)

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | SPECIFICATION FORMAT | FILES | SEE ALSO | HISTORY

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