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

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

       #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

       The tzset() function initializes the tzname variable from the TZ  envi-
       ronment  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 time-
       zone  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:


       The  dst  string  and offset specify the name and offset for the corre-
       sponding daylight saving timezone.  If the offset is  omitted,  it  de-
       faults 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 rep-
              resented; 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

       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:

       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  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     If this variable is set its value takes precedence over the sys-
              tem configured timezone.

       TZDIR  If this variable is set its value takes precedence over the sys-
              tem configured timezone database directory path.

              The system timezone file.

              The system timezone database directory.

              When a TZ string includes a dst timezone without  anything  fol-
              lowing  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 config-
       urable when glibc is compiled.

       For an  explanation  of  the  terms  used  in  this  section,  see  at-

       |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, other-
       wise the daylight saving time version.

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

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                                  2017-09-15                          TZSET(3)
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