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TZSET(3) Linux Programmer’s Manual TZSET(3)
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;
DESCRIPTION
The tzset() function initializes the tzname variable from the TZ environment vari-
able. This function is automatically called by the other time conversion functions
that depend on the time zone. In a SysV-like environment it will also set the
variables timezone (seconds West of GMT) and daylight (0 if this time zone does not
have any daylight savings time rules, nonzero if there is a time during the year
when daylight savings time applies).
If the TZ variable does not appear in the environment, the tzname variable is ini-
tialized 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 NULL 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 time zone:
std offset
The std string specifies the name of the time zone and must be three or more alpha-
betic 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 time zone is west of the Prime Meridian and nega-
tive 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 time zone, as described above. The dst string and offset specify the name
and offset for the corresponding daylight savings time zone. If the offset is
omitted, it defaults to one hour ahead of standard time.
The start field specifies when daylight savings 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.
The third format specifies that the time zone information should be read from a
file:
:[filespec]
If the file specification filespec is omitted, the time zone 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 time zone information from. If
filespec does not begin with a ‘/’, the file specification is relative to the sys-
tem timezone directory.
FILES
The system time zone 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 time zone file
posixrules rules for POSIX-style TZ’s
Often /etc/localtime is a symlink to the file localtime or to the correct time zone
file in the system time zone directory.
CONFORMING TO
SVID 3, POSIX, BSD 4.3
NOTES
Note that the variable daylight does not indicate that daylight savings 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.
BSD4.3 had a routine char *timezone(zone,dst) that returned the name of the time
zone corresponding to its first argument (minutes West of GMT). If the second argu-
ment was 0, the standard name was used, otherwise the daylight savings time ver-
sion.
SEE ALSO
date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
2001-11-13 TZSET(3)
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