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 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 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 num-
       ber 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 argu-
       ment (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)