tzset, tzname, timezone, daylight — initialize time conversion information
#include <time.h>
void
tzset( |
void) ; |
extern char *tzname[2]; extern long timezone; extern int daylight;
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 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 saving time
rules, non-zero 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 timezone
directory.)
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 time zone:
std offset
The std
string
specifies the name of the time zone 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 time zone 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
time zone, as described above. The dst
string and offset
specify the name and
offset for the corresponding daylight saving time zone. 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:
n
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.
m
.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 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 system
timezone directory.
Here's an example, once more for New Zealand:
TZ=":Pacific/Auckland"
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.
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 time zone corresponding to its first argument (minutes
West of GMT). 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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