tzset, tzname, timezone, daylight - initialize time conversion information
#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 since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _SVID_SOURCE
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
characters 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"
- 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.
- /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.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| tzset () |
Thread safety |
MT-Safe env locale |
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
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.
date(1),
gettimeofday(2),
time(2),
ctime(3),
getenv(3),
tzfile(5)