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TZSET(3)		 BSD Library Functions Manual		      TZSET(3)

NAME
     tzset — initialize time conversion information

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     void
     tzset(void);

DESCRIPTION
     The tzset() function uses the value of the environment variable TZ to set
     time conversion information used by localtime(3).	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 by
     localtime(3).  If TZ appears in the environment but its value is a null
     string, Coordinated Universal Time (UTC) is used (without leap second
     correction).  If TZ appears in the environment and its value is not a
     null string:

     -	 if the value begins with a colon, it is used as a pathname of a file
	 from which to read the time conversion information;

     -	 if the value does not begin with a colon, it is first used as the
	 pathname of a file from which to read the time conversion informa‐
	 tion, and, if that file cannot be read, is used directly as a speci‐
	 fication of the time conversion information.

     When TZ is used as a pathname, if it begins with a slash, it is used as
     an absolute pathname; otherwise, it is used as a pathname relative to
     /usr/share/zoneinfo.  The file must be in the format specified in
     tzfile(5).

     When TZ is used directly as a specification of the time conversion infor‐
     mation, it must have the following syntax (spaces inserted for clarity):

	   stdoffset[dst[offset][,rule]]

     where:
     std and dst  Three or more bytes that are the designation for the stan‐
		  dard (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 fol‐
		  lowing 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:
		  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 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 Sun‐
				  day.
		  The time has the same format as offset except that no lead‐
		  ing sign “-” or “+” is allowed.  The default, if time is not
		  given, is 02:00:00.
     If no rule is present in TZ, the rules specified by the tzfile(5) format
     file posixrules in /usr/share/zoneinfo 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.

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

FILES
     /etc/localtime		     local time zone file
     /usr/share/zoneinfo	     time zone information directory
     /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
     /usr/share/zoneinfo/posixrules.

SEE ALSO
     ctime(3), getenv(3), strftime(3), time(3), tzfile(5)

STANDARDS
     The tzset() function conforms to IEEE Std 1003.1-1988 (“POSIX.1”).

BSD				 April 1, 2001				   BSD
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