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 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 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 cannot be interpreted using any of the formats speci-
       fied 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 following 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	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 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  algo-
       rithm (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, 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 3.25 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/.

								    2010-02-25								  TZSET(3)