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TZSET
Section: Linux Programmer's Manual (3)
Updated: 2010-02-25
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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.
February 29 is never counted even in leap years.
- n
-
This specifies the Julian day with n between 1 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 aq/aq, 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.
SEE ALSO
date(1),
gettimeofday(2),
time(2),
ctime(3),
getenv(3),
tzfile(5)
COLOPHON
This page is part of release 3.27 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- FILES
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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