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

NAME
     tzset - initialize time conversion information



SYNOPSIS
     #include <time.h>

     void
     tzset(void);

DESCRIPTION
     tzset() uses the value of the environment variable TZ to set time conver-
     sion information used by localtime(3).  If TZ does not appear in the en-
     vironment, the best available approximation to local wall clock time, as
     specified by the tzfile(5) format file localtime in the system time con-
     version information directory, is used by localtime(3).

     If TZ appears in the environment but its value is a null string, Coordi-
     nated Universal Time (UTC) is used (without leap second correction).

     If TZ appears in the environment and its value begins with a colon, it is
     used as a pathname of a file from which to read the time conversion in-
     formation.

     If TZ appears in the environment and its 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 a
     system time conversion information directory.  The file must be in the
     format specified in tzfile(5).

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

           std offset [dst [offset]] [, rule]

     Where:

     std and dst  Three or more bytes that are the designation for the stan-
                  dard (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.  offset has the form (spaces
                  inserted for clarity):

                        hh [: mm [: ss]]

                  The minutes (mm) and seconds (ss) are optional.  The hour
                  (hh) is required and may be a single digit.  The offset fol-
                  lowing 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.  rule
                  has the form (spaces added for clarity):

                        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.

                  The format of date is one of the following (spaces added for
                  clarity):

                  J n          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 re-
                               fer 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.

                  M m . n . d  Day d (1 <= d <= 6) of week n (1 <= n <= 5) of
                               month m (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 dth day occurs.
                               Day zero is Sunday.

                  The time has the same format as offset except that no lead-
                  ing sign (``-'' or ``+'') is allowed.  The default, if time
                  is not given, is 02:00:00.

     If no rule is present in TZ, the rules specified by the tzfile(5) format
     file posixrules in the system time conversion information directory 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
     /usr/share/zoneinfo             time zone information directory
     /etc/localtime                  local time zone file
     /usr/share/zoneinfo/posixrules  used with POSIX-style TZs
     /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)

OpenBSD 2.6                      May 24, 1999                                2