NAME    [Toc]    [Back]

     tzset - initialize time conversion information

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <time.h>

     void
     tzset(void);

DESCRIPTION    [Toc]    [Back]

     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, 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
     /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.
                  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 /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    [Toc]    [Back]

     /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    [Toc]    [Back]

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

STANDARDS    [Toc]    [Back]

     The tzset() function conforms to IEEE Std 1003.1-1988 (``POSIX.1'').

BSD                              April 1, 2001                             BSD