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TZSET(3)		 BSD 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 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, 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 charac‐
     ter of the pathname is a slash (‘/’) it is used as an absolute pathname;
     otherwise, it is used as a pathname relative to the system time conver‐
     sion 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 inter‐
     preted 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 infor‐
     mation, 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 fol‐
			lows 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
			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 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.

			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 Sun‐
				 day.

				 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 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/GMT	     for UTC leap seconds

     If the file /usr/share/zoneinfo/GMT 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.

BSD			       November 17, 1993			   BSD
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