tzset

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
       tzname: _POSIX_C_SOURCE
       timezone, daylight: _XOPEN_SOURCE
           || /* Glibc since 2.19: */ _DEFAULT_SOURCE
           || /* Glibc versions <= 2.19: */ _SVID_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, 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
       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
       /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
       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
       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES
       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 5.09 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/.



                                   2017-09-15                           TZSET(3)