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NAME
tzset, tzname, timezone, daylight - initialize time conversion information
LIBRARY
Standard C library (libc, -lc)
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
tzname:
_POSIX_C_SOURCE
timezone, daylight:
_XOPEN_SOURCE
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_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, past, present, or future when
daylight saving time applies).
If the TZ variable does not appear in the environment, the system timezone is used. The
system timezone is configured by copying, or linking, a file in the tzfile(5) format to
/etc/localtime. A timezone database of these files may be located in the system timezone
directory (see the FILES section below).
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, then Coordinated Universal
Time (UTC) is used.
The value of TZ can be one of two formats. The first format is a string of characters
that directly represent the timezone to be used:
std offset[dst[offset][,start[/time],end[/time]]]
There are no spaces in the specification. The std string specifies an abbreviation for
the timezone and must be three or more alphabetic characters. When enclosed between the
less-than (<) and greater-than (>) signs, the character set is expanded to include the
plus (+) sign, the minus (-) sign, and digits. 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 00 and 59:
[+|-]hh[:mm[:ss]]
The dst string and offset specify the name and offset for the corresponding daylight
saving timezone. If the offset is omitted, it defaults 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 second format specifies that the timezone information should be read from a file:
:[filespec]
If the file specification filespec is omitted, or its value cannot be interpreted, then
Coordinated Universal Time (UTC) is used. 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. If the
colon is omitted each of the above TZ formats will be tried.
Here's an example, once more for New Zealand:
TZ=":Pacific/Auckland"
ENVIRONMENT
TZ If this variable is set its value takes precedence over the system configured
timezone.
TZDIR If this variable is set its value takes precedence over the system configured
timezone database directory path.
FILES
/etc/localtime
The system timezone file.
/usr/share/zoneinfo/
The system timezone database directory.
/usr/share/zoneinfo/posixrules
When a TZ string includes a dst timezone without anything following it, then this
file is used for the start/end rules. It is in the tzfile(5) format. By default,
the zoneinfo Makefile hard links it to the America/New_York tzfile.
Above are the current standard file locations, but they are configurable when glibc is
compiled.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────────────┬───────────────┬────────────────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────────────┼───────────────┼────────────────────┤
│tzset() │ Thread safety │ MT-Safe env locale │
└────────────────────────────────────────────────────┴───────────────┴────────────────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
NOTES
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)