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Linux 2.6 - man page for tzset (linux section 3)

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

NAME
       tzset, tzname, timezone, daylight - initialize time conversion information

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 environment 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  sav-
       ing 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 variable 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/localtime 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 can-
       not be interpreted using any of the formats specified 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 Coordinated 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 saving 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  fol-
       lowing formats:

       Jn     This  specifies  the  Julian  day  with  n between 1 and 365.  February 29 is never
	      counted even in leap years.

       n      This specifies the Julian day with n between 1 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 speci-
       fies 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 environment 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 correct 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 get-
       timeofday(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, otherwise 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 3.27 of the Linux man-pages project.  A  description  of  the
       project,   and	information  about  reporting  bugs,  can  be  found  at  http://www.ker-
       nel.org/doc/man-pages/.

					    2010-02-25					 TZSET(3)


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