newtzset(3)                Library Functions Manual                newtzset(3)

NAME
       tzset - initialize time conversion information

SYNOPSIS
       #include <time.h>

       timezone_t tzalloc(char const *TZ);

       void tzfree(timezone_t tz);

       void tzset(void);

       cc ... -ltz

DESCRIPTION
       The tzalloc function allocates and returns a timezone object described
       by TZ.  If TZ is not a valid timezone description, or if the object
       cannot be allocated, tzalloc returns a null pointer and sets errno.

       The tzfree function frees a timezone object tz, which should have been
       successfully allocated by tzalloc.  This invalidates any tm_zone
       pointers that tz was used to set.

       The tzset function acts like tzalloc(getenv("TZ")), except it saves any
       resulting timezone object into internal storage that is accessed by
       localtime, localtime_r, and mktime.  The anonymous shared timezone
       object is freed by the next call to tzset.  If the implied call to
       tzalloc fails, tzset falls back on Universal Time (UT).

       If TZ is null, 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.  If TZ is the empty
       string, UT is used, with the abbreviation "UTC" and without leap second
       correction; please see newctime(3) for more about UT, UTC, and leap
       seconds.  If TZ is nonnull and nonempty:

              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 the alternative (dst, such
                             as daylight saving time) time zone.  Only std is
                             required; if dst is missing, then daylight saving
                             time does not apply in this locale.  Upper- and
                             lowercase letters are explicitly allowed.  Any
                             characters except a leading colon (:), digits,
                             comma (,), ASCII minus (-), ASCII plus (+), and
                             NUL bytes are allowed.  Alternatively, a
                             designation can be surrounded by angle brackets <
                             and >; in this case, the designation can contain
                             any characters other than > and NUL.

              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, daylight saving 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
                             daylight saving time.  The rule has the form:

                                    date/time,date/time

                             where the first date describes when the change
                             from standard to daylight saving 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
                             "+").  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 timezone;
       they use some of the extensions to POSIX.

       EST5   stands for US Eastern Standard Time (EST), 5 hours behind UT,
              without daylight saving.

       <+12>-12<+13>,M11.1.0,M1.2.1/147
              stands for Fiji time, 12 hours ahead of UT, springing forward on
              November's first Sunday at 02:00, and falling back on January's
              second Monday at 147:00 (i.e., 03:00 on the first Sunday on or
              after January 14).  The abbreviations for standard and daylight
              saving time are "+12" and "+13".

       IST-2IDT,M3.4.4/26,M10.5.0
              stands for Israel Standard Time (IST) and Israel Daylight Time
              (IDT), 2 hours ahead of UT, springing forward on March's fourth
              Thursday 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.

       <-04>4<-03>,J1/0,J365/25
              stands for permanent daylight saving time, 3 hours behind UT
              with abbreviation "-03".  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 <-04> is a placeholder.

       <-03>3<-02>,M3.5.0/-2,M10.5.0/-1
              stands for time in western Greenland, 3 hours behind UT, where
              clocks follow the EU rules of springing forward on March's last
              Sunday at 01:00 UT (-02:00 local time, i.e., 22:00 the previous
              day) and falling back on October's last Sunday at 01:00 UT
              (-01:00 local time, i.e., 23:00 the previous day).  The
              abbreviations for standard and daylight saving time are "-03"
              and "-02".

       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 daylight saving
       time offsets from UT 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.

FILES
       /usr/share/zoneinfo             timezone information directory
       /usr/share/zoneinfo/localtime   local timezone file
       /usr/share/zoneinfo/posixrules  default DST rules (obsolete,
                                       and can cause bugs if present)
       /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
       getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)

Time Zone Database                                                 newtzset(3)