tzset(3)                                              Library Functions Manual                                             tzset(3)

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

       Standard C library (libc, -lc)

       #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)):



       timezone, daylight:
               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _SVID_SOURCE

       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 character set is ex‐
       panded to include the plus (+) sign, the minus (-) sign, and digits.  The offset string immediately follows std  and  speci‐
       fies  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 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 hap‐
       pen at the default time of 02:00:00:


       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 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     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.

              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  tz‐

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

       For an explanation of the terms used in this section, see attributes(7).

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

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

       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  ver‐

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

Linux man-pages 6.03                                         2023-02-05                                                    tzset(3)