NAME

     tzset - initialize time conversion information

SYNOPSIS
     void tzset()

     cc ... -ltz

DESCRIPTION
     Tzset uses the value of the environment variable TZ to set
     time conversion information used by localtime.  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 localtime in the system time
     conversion information directory, is used by localtime.  If
     TZ appears in the environment but its value is a null
     string, Coordinated Universal Time (UTC) is used (without
     leap second correction).  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 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 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.

                         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 no leading 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 and cannot be interpreted as a direct
     specification, UTC is used.

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

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

SEE ALSO
     getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)
