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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).

       ┌──────────┬───────────────┬────────────────────┐
       │InterfaceAttributeValue              │
       ├──────────┼───────────────┼────────────────────┤
       │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 5.02 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)