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TZSET(3)		   Linux Programmer's Manual		      TZSET(3)

NAME
       tzset, tzname, timezone,	daylight - initialize time conversion informa-
       tion

SYNOPSIS
       #include	<time.h>

       void tzset (void);

       extern char *tzname[2];
       extern long timezone;
       extern int daylight;

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       tzset():	_POSIX_C_SOURCE	>= 1 ||	_XOPEN_SOURCE || _POSIX_SOURCE
       tzname: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE ||	_POSIX_SOURCE
       timezone: _SVID_SOURCE || _XOPEN_SOURCE
       daylight: _SVID_SOURCE || _XOPEN_SOURCE

DESCRIPTION
       The tzset() function initializes	the tzname variable from the TZ	 envi-
       ronment	variable.   This function is automatically called by the other
       time conversion functions that depend on	the timezone.  In a  System-V-
       like environment, it will also set the variables	timezone (seconds West
       of UTC) and daylight (to	0 if this timezone does	not have any  daylight
       saving  time  rules,  or	 to nonzero if there is	a time during the year
       when daylight saving time applies).

       If the TZ variable does not appear in the environment, the tzname vari-
       able  is	 initialized  with  the	best approximation of local wall clock
       time, as	specified by the tzfile(5)-format file localtime found in  the
       system  timezone	 directory (see	below).	 (One also often sees /etc/lo-
       caltime used here, a symlink to the right file in the  system  timezone
       directory.)

       If  the	TZ  variable  does  appear in the environment but its value is
       empty or	its value cannot be interpreted	using any of the formats spec-
       ified below, Coordinated	Universal Time (UTC) is	used.

       The  value of TZ	can be one of three formats.  The first	format is used
       when there is no	daylight saving	time in	the local timezone:

	      std offset

       The std string specifies	the name of the	timezone and must be three  or
       more  alphabetic	characters.  The offset	string immediately follows std
       and specifies the time value to be added	to the local time to get Coor-
       dinated	Universal  Time	 (UTC).	  The  offset is positive if the local
       timezone	is west	of the Prime Meridian and negative if it is east.  The
       hour must be between 0 and 24, and the minutes and seconds 0 and	59.

       The second format is used when there is daylight	saving time:

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

       There  are  no spaces in	the specification.  The	initial	std and	offset
       specify the standard timezone, as described above.  The dst string  and
       offset  specify the name	and offset for the corresponding daylight sav-
       ing timezone.  If the offset is omitted,	it default to one  hour	 ahead
       of standard time.

       The  start  field  specifies when daylight saving time goes into	effect
       and the end field specifies when	the change is made  back  to  standard
       time.  These fields may have the	following formats:

       Jn     This  specifies  the  Julian day with n between 1	and 365.  Leap
	      days are not counted.  In	this format, February 29 can't be rep-
	      resented;	February 28 is day 59, and March 1 is always day 60.

       n      This  specifies  the  zero-based Julian day with n between 0 and
	      365.  February 29	is counted in leap years.

       Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w  <=	5)  of
	      month m (1 <= m <= 12).  Week 1 is the first week	in which day d
	      occurs and week 5	is the last week in which day d	occurs.	 Day 0
	      is a Sunday.

       The  time  fields  specify when,	in the local time currently in effect,
       the change to the other	time  occurs.	If  omitted,  the  default  is
       02:00:00.

       Here  is	 an example for	New Zealand, where the standard	time (NZST) is
       12 hours	ahead of UTC, and daylight saving time (NZDT), 13 hours	 ahead
       of  UTC,	 runs  from the	first Sunday in	October	to the third Sunday in
       March, and the changeovers happen at the	default	time of	02:00:00:

	   TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"

       The third format	specifies that the timezone information	should be read
       from a file:

	      :[filespec]

       If the file specification filespec is omitted, the timezone information
       is read from the	file localtime in the system timezone directory, which
       nowadays	 usually  is  /usr/share/zoneinfo.   This file is in tzfile(5)
       format.	If filespec is given, it  specifies  another  tzfile(5)-format
       file to read the	timezone information from.  If filespec	does not begin
       with a '/', the file specification is relative to the  system  timezone
       directory.

       Here's an example, once more for	New Zealand:

	   TZ=":Pacific/Auckland"

FILES
       The  system  timezone  directory	 used  depends on the (g)libc version.
       Libc4 and libc5 use /usr/lib/zoneinfo, and, since libc-5.4.6, when this
       doesn't	work, will try /usr/share/zoneinfo.  Glibc2 will use the envi-
       ronment variable	TZDIR, when that exists.  Its default depends  on  how
       it was installed, but normally is /usr/share/zoneinfo.

       This timezone directory contains	the files
       localtime      local timezone file
       posixrules     rules for	POSIX-style TZ's

       Often  /etc/localtime is	a symlink to the file localtime	or to the cor-
       rect timezone file in the system	timezone directory.

CONFORMING TO
       SVr4, POSIX.1-2001, 4.3BSD.

NOTES
       Note that the variable daylight does not	indicate that daylight	saving
       time  applies  right now.  It used to give the number of	some algorithm
       (see the	variable tz_dsttime in gettimeofday(2)).  It has been obsolete
       for many	years but is required by SUSv2.

       4.3BSD  had a function char *timezone(zone, dst)	that returned the name
       of the timezone corresponding to	its first argument  (minutes  West  of
       UTC).  If the second argument was 0, the	standard name was used,	other-
       wise the	daylight saving	time version.

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

COLOPHON
       This page is part of release 3.53 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at http://www.kernel.org/doc/man-pages/.

				  2012-03-25			      TZSET(3)

NAME | SYNOPSIS | DESCRIPTION | FILES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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