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 link-
       ing,  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 repre-
              sented; 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 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:

           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 4.15 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)