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);

       /* Optional and obsolescent:  */
       extern char *tzname[];
       extern long timezone;
       extern int daylight;

       cc ... -ltz

DESCRIPTION
       The tzalloc function allocates and returns a timezone object described
       by TZ.

       If TZ is a null pointer, tzalloc uses 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.

       If TZ is the empty string, tzalloc uses Universal Time (UT), 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 contents are used as a pathname, a pathname beginning with “/”
       is used as-is; otherwise the pathname is relative to a system time
       conversion information directory.  In a privileged program the pathname
       must be relative.  Relative pathnames must not contain “..” components.
       For the purpose of these checks, a file name beginning with “/” is
       considered to be relative if it is the localtime file's name, or if it
       starts with the system timezone directory's name followed by one more
       more slashes.  The file must be a regular file 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:

              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.  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 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.

       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 rule 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 TZ specifies daylight saving time but does not specify a rule, the
       rule typically defaults to the current US daylight-saving rule,
       although such a default is not guaranteed and is incorrect for many
       locations outside the US.  Therefore, if a TZ string directly specifies
       a timezone with daylight saving time, it should specify the daylight
       saving rule explicitly.

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

       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
       getenv fails, tzset acts like tzalloc(nullptr); if the implied call to
       tzalloc fails, tzset falls back on UT.

       As a side effect, the tzset function sets some external variables if
       the platform defines them.  It sets tzname[0] and tzname[1] to pointers
       to strings that are time zone abbreviations to be used with standard
       and daylight saving time, respectively.  It also sets timezone to be
       the number of seconds that standard time is west of the Prime Meridian,
       and daylight to be zero if daylight saving time is never in effect,
       non-zero otherwise.

RETURN VALUE
       If successful, the tzalloc function returns a nonnull pointer to the
       newly allocated object.  Otherwise, it returns a null pointer and sets
       errno.  The tzfree function does not modify errno.

ERRORS
       EOVERFLOW
              TZ directly specifies time conversion information, and contains
              an integer out of machine range or a time zone abbreviation that
              is too long for this platform.

       The tzalloc function may also fail and set errno for any of the errors
       specified for the routines access(2), close(2), malloc(3), open(2), and
       read(2).

FILES
       /etc/localtime           local timezone file
       /usr/share/zoneinfo      timezone directory
       /usr/share/zoneinfo/GMT  for UTC leap seconds

       If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
       /usr/share/zoneinfo/GMT0 if present.

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

NOTES
       Portable code should not rely on the contents of the external variables
       tzname, timezone and daylight as their contents are unspecified (and do
       not make sense in general) when a geographical TZ is used.  In
       multithreaded applications behavior is undefined if one thread accesses
       one of these variables while another thread invokes tzset.  A future
       version of POSIX is planned to remove these variables; callers can
       instead use the tm_gmtoff and tm_zone members of struct tm, or use
       strftime with "%z" or "%Z".

Time Zone Database                                                 newtzset(3)
