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TZSET(3)                 BSD Library Functions Manual                 TZSET(3)

NAME
     tzset -- initialize time conversion information

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     void
     tzset(void);

DESCRIPTION
     The tzset() function uses the value of the environment variable TZ to set
     time conversion information used by localtime(3).  If TZ does not appear
     in the environment, the best available approximation to local wall clock
     time, as specified by the tzfile(5) format file /etc/localtime is used by
     localtime(3).  If TZ appears in the environment but its value is a null
     string, Universal Time (UT) is used, with the abbreviation "UTC" and
     without leap second correction; please see ctime(3).  If TZ appears in
     the environment and its value is not a null string:

     -   if the value begins with a colon, it is used as a pathname of a file
         from which to read the time conversion information;

     -   if the value does not begin with a colon, it is first used as the
         pathname of a file from which to read the time conversion
         information, and, if that file cannot be read, is used directly as a
         specification of the time conversion information.

     When TZ is used as a pathname, if it begins with a slash, it is used as
     an absolute pathname; otherwise, it is used as a pathname relative to
     /usr/share/zoneinfo.  The file must be in the format specified in
     tzfile(5).

     When TZ is used directly as a specification of the time conversion
     information, it must have the following syntax (spaces inserted for
     clarity):

           stdoffset[dst[offset][,rule]]

     where:
     std and dst  Three or more bytes that are the designation for the
                  standard (std) or summer (dst) time zone.  Only std is
                  required; if dst is missing, then summer time does not apply
                  in this locale.  Upper- and lowercase letters are explicitly
                  allowed.  Any characters except a leading colon (:), digits,
                  comma (,), minus (-), plus (+), and ASCII NUL are allowed.
     offset       Indicates the value one must add to the local time to arrive
                  at Coordinated Universal Time.  The offset has the form:

                        hh[:mm[:ss]]

                  The minutes (mm) and seconds (ss) are optional.  The hour
                  (hh) is required and may be a single digit.  The offset
                  following std is required.  If no offset follows dst, summer
                  time is assumed to be one hour ahead of standard time.  One
                  or more digits may be used; the value is always interpreted
                  as a decimal number.  The hour must be between zero and 24,
                  and the minutes (and seconds) -- if present -- between zero
                  and 59.  If preceded by a "-" the time zone shall be east of
                  the Prime Meridian; otherwise it shall be west (which may be
                  indicated by an optional preceding "+").
     rule         Indicates when to change to and back from summer time.  The
                  rule has the form:

                        date/time,date/time

                  where the first date describes when the change from standard
                  to summer time occurs and the second date describes when the
                  change back happens.  Each time field describes when, in
                  current local time, the change to the other time is made.
                  As an extension to POSIX, daylight saving is assumed to be
                  in effect all year if it begins January 1 at 00:00 and ends
                  December 31 at 24:00 plus the difference between daylight
                  saving and standard time, leaving no room for standard time
                  in the calendar.  The format of date is one of the
                  following:
                  Jn              The Julian day n (1 <= n <= 365).  Leap days
                                  are not counted; that is, in all years --
                                  including leap years -- February 28 is day
                                  59 and March 1 is day 60.  It is impossible
                                  to explicitly refer to the occasional
                                  February 29.
                  n               The zero-based Julian day (0 <= n <= 365).
                                  Leap days are counted, and it is possible to
                                  refer to February 29.
                  Mm.n.d          The d'th day (0 <= d <= 6) of week n of
                                  month m of the year (1 <= n <= 5, 1 <= m
                                  <= 12, where week 5 means "the last d
                                  day in month m" which may occur in either
                                  the fourth or the fifth week).  Week 1 is
                                  the first week in which the d'th day occurs.
                                  Day zero is Sunday.
                  The time has the same format as offset except that POSIX
                  does not allow a leading sign "-" or "+" is allowed.  As an
                  extension to POSIX, the hours part of time can range from
                  -167 through 167; this allows for unusual rules such as "the
                  Saturday before the first Sunday of March".  The default, if
                  time is not given, is 02:00:00.

     Here are some examples of TZ values that directly specify the time zone
     rules; they use some of the extensions to POSIX.

     EST5    stands for US eastern time (EST), 5 hours behind UTC, without
             daylight saving.

     FJT-12FJST,M10.3.1/146,M1.3.4/75
             stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours
             ahead of UTC, springing forward on October's third Monday at
             146:00 (i.e., 02:00 on the first Sunday on or after October 21),
             and falling back on January's third Thursday at 75:00 (i.e.,
             03:00 on the first Sunday on or after January 18).

     IST-2IDT,M3.4.4/26,M10.5.0
             stands for Israel Standard Time (IST) and Israel Daylight Time
             (IDT), 2 hours ahead of UTC, springing forward on March's fourth
             Tuesday at 26:00 (i.e., 02:00 on the first Friday on or after
             March 23), and falling back on October's last Sunday at 02:00.

     WART4WARST,J1/0,J365/25
             stands for Western Argentina Summer Time (WARST), 3 hours behind
             UTC.  There is a dummy fall-back transition on December 31 at
             25:00 daylight saving time (i.e., 24:00 standard time, equivalent
             to January 1 at 00:00 standard time), and a simultaneous spring-
             forward transition on January 1 at 00:00 standard time, so
             daylight saving time is in effect all year and the initial WART
             is a placeholder.

     WGT3WGST,M3.5.0/-2,M10.5.0/-1
             stands for Western Greenland time (WGT) and Western Greenland
             Summer Time (WGST), 3 hours behind UTC, where clocks follow the
             EU rules of springing forward on March's last Sunday at 01:00 UTC
             (-02:00 local time) and falling back on October's last Sunday at
             01:00 UTC (-01:00 local time).

     If no rule is present in TZ, the rules specified by the tzfile(5) format
     file posixrules in /usr/share/zoneinfo are used, with the standard and
     summer time offsets from UTC replaced by those specified by the offset
     values in TZ.

     For compatibility with System V Release 3.1, a semicolon (;) may be used
     to separate the rule from the rest of the specification.

     If the TZ environment variable does not specify a tzfile(5) format file
     and cannot be interpreted as a direct specification, UTC is used.

FILES
     /etc/localtime                  local time zone file
     /usr/share/zoneinfo             time zone information directory
     /usr/share/zoneinfo/posixrules  used with POSIX-style TZ's
     /usr/share/zoneinfo/GMT         for UTC leap seconds

     If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
     /usr/share/zoneinfo/posixrules.

SEE ALSO
     ctime(3), getenv(3), strftime(3), time(3), tzfile(5)

STANDARDS
     The tzset() function conforms to IEEE Std 1003.1-1988 ("POSIX.1").

BSD                           September 20, 2013                           BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | FILES | SEE ALSO | STANDARDS