tzset, tzname, timezone, daylight - initialize time conversion
void tzset (void);
extern char *tzname;
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
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
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:
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
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 third format specifies that the timezone information should be read
from a file:
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
Here’s an example, once more for New Zealand:
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.
SVr4, POSIX.1-2001, 4.3BSD.
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.
date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
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