timezone(3) — Linux manual page


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

NAME         top

       tzset, tzname, timezone, daylight - initialize time conversion infor‐

SYNOPSIS         top

       #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
       tzname: _POSIX_C_SOURCE
       timezone, daylight: _XOPEN_SOURCE
           || /* Glibc since 2.19: */ _DEFAULT_SOURCE
           || /* Glibc versions <= 2.19: */ _SVID_SOURCE

DESCRIPTION         top

       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, past, present or future when daylight saving time applies).

       If the TZ variable does not appear in the environment, the system
       timezone is used.  The system timezone is configured by copying, or
       linking, a file in the tzfile(5) format to /etc/localtime.  A
       timezone database of these files may be located in the system
       timezone directory (see the FILES section below).

       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, then Coordinated Universal Time (UTC) is used.

       The value of TZ can be one of two formats.  The first format is a
       string of characters that directly represent the timezone to be used:

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

       There are no spaces in the specification.  The std string specifies
       an abbreviation for the timezone and must be three or more alphabetic
       characters.  When enclosed between the less-than (<) and greater-than
       (>) signs, the characters set is expanded to include the plus (+)
       sign, the minus (-) sign, and digits.  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 00 and 59:


       The dst string and offset specify the name and offset for the corre‐
       sponding daylight saving timezone.  If the offset is omitted, it
       defaults 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
              represented; February 28 is day 59, and March 1 is always day

       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

       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 Sun‐
       day in March, and the changeovers happen at the default time of


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


       If the file specification filespec is omitted, or its value cannot be
       interpreted, then Coordinated Universal Time (UTC) is used.  If file‐
       spec 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.
       If the colon is omitted each of the above TZ formats will be tried.

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


ENVIRONMENT         top

       TZ     If this variable is set its value takes precedence over the
              system configured timezone.

       TZDIR  If this variable is set its value takes precedence over the
              system configured timezone database directory path.

FILES         top

              The system timezone file.

              The system timezone database directory.

              When a TZ string includes a dst timezone without anything
              following it, then this file is used for the start/end rules.
              It is in the tzfile(5) format.  By default, the zoneinfo
              Makefile hard links it to the America/New_York tzfile.

       Above are the current standard file locations, but they are
       configurable when glibc is compiled.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface Attribute     Value              │
       │tzset()   │ Thread safety │ MT-Safe env locale │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES         top

       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         top

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

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
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       latest version of this page, can be found at

                                 2017-09-15                         TZSET(3)