# tzset

## NAME



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


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## 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 >= 2.19: */ _DEFAULT_SOURCE
                   || /* glibc <= 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).
    
           The **tzset**() function initializes these variables to unspecified
           values if this timezone is a geographical timezone like
           "America/New_York" (see below).
    
           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.
    
           A nonempty value of **TZ** can be one of two formats, either of which
           can be preceded by a colon which is ignored.  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 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:
    
           J _n_     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.
    
           M _m_._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.  They use the same
           format as _offset_ except that the hour can be in the range [**-167** ,
           **167**] to represent times before and after the named day.  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 September's last Sunday, at the
           default time 02:00:00, to April's first Sunday at 03:00:00.
    
               TZ="NZST-12:00:00NZDT-13:00:00,M9.5.0,M4.1.0/3"
    
           The second —or "geographical"— format specifies that the timezone
           information should be read from a file:
    
               filespec
    
           The _filespec_ specifies a 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 specified file cannot be read or interpreted,
           Coordinated Universal Time (UTC) is used; however, applications
           should not depend on random _filespec_ values standing for UTC, as
           **TZ** formats may be extended in the future.
    
           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 │
           └───────────────────────────┴───────────────┴────────────────────┘
    


## STANDARDS



           POSIX.1-2024.
    


## CAVEATS



           Because the values of _tzname_ , _timezone_ , and _daylight_ are often
           unspecified, and accessing them can lead to undefined behavior in
           multithreaded applications, code should instead obtain time zone
           offset and abbreviations from the _tm_gmtoff_ and _tm_zone_ members of
           the broken-down time structure tm(3type).
    


## BUGS



           Since this function does not report errors, there's no way to know
           if the value of TZ represents a valid time zone.
    


* * *