tzset, tzname, timezone, daylight - initialize time conversion information
#include <time.h> void tzset (void); extern char *tzname; extern long timezone; extern int daylight;
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
daylight: _XOPEN_SOURCE || /* Glibc since
2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */
The tzset() function initializes the
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:
There are no spaces in the specification. The
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 characters set is expanded to include the
plus (+) sign, the minus (-) sign, and digits. The
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:
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.
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:
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.
This specifies the zero-based Julian day with
n between 0
and 365. February 29 is counted in leap years.
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
occurs and week 5 is the last week in which day
d occurs. Day 0
is a Sunday.
time fields specify when, in the local time currently in
effect, the change to the other time occurs. If omitted, the default is
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:
The second format specifies that the timezone information should be read from a file:
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
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
Here's an example, once more for New Zealand:
If this variable is set its value takes precedence over the system configured timezone.
If this variable is set its value takes precedence over the system configured timezone database directory path.
The system timezone file.
The system timezone database directory.
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
Above are the current standard file locations, but they are configurable when glibc is compiled.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
4.3BSD had a function char
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.
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