Ubuntu Manpage: tzset, tzname, timezone, daylight - initialize time conversion information

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NAME

       tzset, tzname, timezone, daylight - initialize time conversion information

SYNOPSIS

       #include <time.h>

       void tzset (void);

       extern char *tzname[2];
       extern long timezone;
       extern int daylight;

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       tzset(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       tzname: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       timezone: _SVID_SOURCE || _XOPEN_SOURCE
       daylight: _SVID_SOURCE || _XOPEN_SOURCE

DESCRIPTION

The tzset() function initializes the tzname variable from the TZ environment variable.
This function is automatically called by the other time conversion functions that depend
on the timezone. In a System-V-like environment, it will also set the variables timezone
(seconds West of UTC) and daylight (to 0 if this timezone does not have any daylight
saving time rules, or to nonzero if there is a time during the year when daylight saving
time applies).

If the TZ variable does not appear in the environment, the tzname variable is initialized
with the best approximation of local wall clock time, as specified by the tzfile(5)-format
file localtime found in the system timezone directory (see below). (One also often sees
/etc/localtime used here, a symlink to the right file in the system timezone directory.)

If the TZ variable does appear in the environment but its value is empty or its value
cannot be interpreted using any of the formats specified below, Coordinated Universal Time
(UTC) is used.

The value of TZ can be one of three formats. The first format is used when there is no
daylight saving time in the local timezone:

std offset

The std string specifies the name of the timezone and must be three or more alphabetic
characters. The offset string immediately follows std and specifies the time value to be
added to the local time to get Coordinated Universal Time (UTC). The offset is positive
if the local timezone is west of the Prime Meridian and negative if it is east. The hour
must be between 0 and 24, and the minutes and seconds 0 and 59.

The second format is used when there is daylight saving time:

std offset dst [offset],start[/time],end[/time]

There are no spaces in the specification. The initial std and offset specify the standard
timezone, as described above. The dst string and offset specify the name and offset for
the corresponding daylight saving timezone. If the offset is omitted, it default to one
hour ahead of standard time.

The start field specifies when daylight saving time goes into effect and the end field
specifies when the change is made back to standard time. These fields may have the
following formats:

Jn This specifies the Julian day with n between 1 and 365. Leap days are not counted.
In this format, February 29 can't be represented; February 28 is day 59, and March
1 is always day 60.

n This specifies the zero-based Julian day with n between 0 and 365. February 29 is
counted in leap years.

Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of month m (1 <= m <=
12). Week 1 is the first week in which day d occurs and week 5 is the last week in
which day d occurs. Day 0 is a Sunday.

The time fields specify when, in the local time currently in effect, the change to the
other time occurs. If omitted, the default is 02:00:00.

Here is an example for New Zealand, where the standard time (NZST) is 12 hours ahead of
UTC, and daylight saving time (NZDT), 13 hours ahead of UTC, runs from the first Sunday in
October to the third Sunday in March, and the changeovers happen at the default time of
02:00:00:

TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"

The third format specifies that the timezone information should be read from a file:

:[filespec]

If the file specification filespec is omitted, the timezone information is read from the
file localtime in the system timezone directory, which nowadays usually is
/usr/share/zoneinfo. This file is in tzfile(5) format. If filespec is given, it
specifies another tzfile(5)-format file to read the timezone information from. If
filespec does not begin with a '/', the file specification is relative to the system
timezone directory.

Here's an example, once more for New Zealand:

TZ=":Pacific/Auckland"

FILES

       The  system  timezone  directory used depends on the (g)libc version.  Libc4 and libc5 use
       /usr/lib/zoneinfo,  and,  since   libc-5.4.6,   when   this   doesn't   work,   will   try
       /usr/share/zoneinfo.   Glibc2  will  use the environment variable TZDIR, when that exists.
       Its default depends on how it was installed, but normally is /usr/share/zoneinfo.

       This timezone directory contains the files
       localtime      local timezone file
       posixrules     rules for POSIX-style TZ's

       Often /etc/localtime is a symlink to the file localtime or to the correct timezone file in
       the system timezone directory.

CONFORMING TO

       SVr4, POSIX.1-2001, 4.3BSD.

NOTES

       Note  that the variable daylight does not indicate that daylight saving time applies right
       now.  It used to give the number  of  some  algorithm  (see  the  variable  tz_dsttime  in
       gettimeofday(2)).  It has been obsolete for many years but is required by SUSv2.

       4.3BSD  had  a  function  char *timezone(zone, dst) that returned the name of the timezone
       corresponding to its first argument (minutes West of UTC).  If the second argument was  0,
       the standard name was used, otherwise the daylight saving time version.

COLOPHON

       This  page  is  part of release 3.54 of the Linux man-pages project.  A description of the
       project,    and    information    about    reporting    bugs,    can    be    found     at
       http://www.kernel.org/doc/man-pages/.

                                            2012-03-25                                   TZSET(3)