tzset(3) — Linux manual page

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

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

NAME         top

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

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 <= 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:

           [+|-]hh[:mm[:ss]]

       The dst string and offset specify the name and offset for the
       corresponding 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 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 second format specifies that the timezone information should
       be read from a file:

           :[filespec]

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

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

           TZ=":Pacific/Auckland"

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

       /etc/localtime
              The system timezone file.

       /usr/share/zoneinfo/
              The system timezone database directory.

       /usr/share/zoneinfo/posixrules
              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
       attributes(7).

       ┌───────────────────────────┬───────────────┬────────────────────┐
       │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 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

                               2021-03-22                       TZSET(3)

Pages that refer to this page: groff(1)grohtml(1)gropdf(1)grops(1)homectl(1)troff(1)gettimeofday(2)ctime(3)timegm(3)localtime(5)tzfile(5)environ(7)anacron(8)hwclock(8)