NEWTZSET(3) Library Functions Manual NEWTZSET(3)
NAME
tzset - initialize time conversion information
SYNOPSIS
void tzset()
cc ... -ltz
DESCRIPTION
Tzset uses the value of the environment variable TZ to set time
conversion information used by localtime. If TZ does not appear in the
environment, the best available approximation to local wall clock time,
as specified by the tzfile(5)-format file localtime in the system time
conversion information directory, is used by localtime. If TZ appears
in the environment but its value is a null string, Universal Time (UT)
is used, with the abbreviation "UTC" and without leap second
correction; please see newctime(3) for more about UT, UTC, and leap
seconds. If TZ appears in the environment and its value is not a null
string:
if the value begins with a colon, it is used as a pathname of a
file from which to read the time conversion information;
if the value does not begin with a colon, it is first used as
the pathname of a file from which to read the time conversion
information, and, if that file cannot be read, is used directly
as a specification of the time conversion information.
When TZ is used as a pathname, if it begins with a slash, it is used as
an absolute pathname; otherwise, it is used as a pathname relative to a
system time conversion information directory. The file must be in the
format specified in tzfile(5).
When TZ is used directly as a specification of the time conversion
information, it must have the following syntax (spaces inserted for
clarity):
stdoffset[dst[offset][,rule]]
Where:
std and dst Three or more bytes that are the designation for
the standard (std) or summer (dst) time zone.
Only std is required; if dst is missing, then
summer time does not apply in this locale.
Upper- and lowercase letters are explicitly
allowed. Any characters except a leading colon
(:), digits, comma (,), minus (-), plus (+), and
ASCII NUL are allowed.
offset Indicates the value one must add to the local
time to arrive at Coordinated Universal Time.
The offset has the form:
hh[:mm[:ss]]
The minutes (mm) and seconds (ss) are optional.
The hour (hh) is required and may be a single
digit. The offset following std is required. If
no offset follows dst, summer time is assumed to
be one hour ahead of standard time. One or more
digits may be used; the value is always
interpreted as a decimal number. The hour must
be between zero and 24, and the minutes (and
seconds) -- if present -- between zero and 59.
If preceded by a ``-'', the time zone shall be
east of the Prime Meridian; otherwise it shall be
west (which may be indicated by an optional
preceding ``+'').
rule Indicates when to change to and back from summer
time. The rule has the form:
date/time,date/time
where the first date describes when the change
from standard to summer time occurs and the
second date describes when the change back
happens. Each time field describes when, in
current local time, the change to the other time
is made. As an extension to POSIX, daylight
saving is assumed to be in effect all year if it
begins January 1 at 00:00 and ends December 31 at
24:00 plus the difference between daylight saving
and standard time, leaving no room for standard
time in the calendar.
The format of date is one of the following:
Jn The Julian day n (1 <= n <= 365). Leap
days are not counted; that is, in all
years -- including leap years --
February 28 is day 59 and March 1 is
day 60. It is impossible to explicitly
refer to the occasional February 29.
n The zero-based Julian day
(0 <= n <= 365). Leap days are
counted, and it is possible to refer to
February 29.
Mm.n.d The d'th day (0 <= d <= 6) of week n of
month m of the year (1 <= n <= 5,
1 <= m <= 12, where week 5 means ``the
last d day in month m'' which may occur
in either the fourth or the fifth
week). Week 1 is the first week in
which the d'th day occurs. Day zero is
Sunday.
The time has the same format as offset except
that POSIX does not allow a leading sign (``-''
or ``+''). As an extension to POSIX, the hours
part of time can range from -167 through 167;
this allows for unusual rules such as "the
Saturday before the first Sunday of March". The
default, if time is not given, is 02:00:00.
Here are some examples of TZ values that directly specify the time zone
rules; they use some of the extensions to POSIX.
EST5 stands for US Eastern Standard Time (EST), 5 hours behind UTC,
without daylight saving.
FJT-12FJST,M10.3.1/146,M1.3.4/75
stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours
ahead of UTC, springing forward on October's third Monday at
146:00 (i.e., 02:00 on the first Sunday on or after October 21),
and falling back on January's third Thursday at 75:00 (i.e.,
03:00 on the first Sunday on or after January 18).
IST-2IDT,M3.4.4/26,M10.5.0
stands for Israel Standard Time (IST) and Israel Daylight Time
(IDT), 2 hours ahead of UTC, springing forward on March's fourth
Thursday at 26:00 (i.e., 02:00 on the first Friday on or after
March 23), and falling back on October's last Sunday at 02:00.
WART4WARST,J1/0,J365/25
stands for Western Argentina Summer Time (WARST), 3 hours behind
UTC. There is a dummy fall-back transition on December 31 at
25:00 daylight saving time (i.e., 24:00 standard time,
equivalent to January 1 at 00:00 standard time), and a
simultaneous spring-forward transition on January 1 at 00:00
standard time, so daylight saving time is in effect all year and
the initial WART is a placeholder.
WGT3WGST,M3.5.0/-2,M10.5.0/-1
stands for Western Greenland Time (WGT) and Western Greenland
Summer Time (WGST), 3 hours behind UTC, where clocks follow the
EU rules of springing forward on March's last Sunday at 01:00
UTC (-02:00 local time) and falling back on October's last
Sunday at 01:00 UTC (-01:00 local time).
If no rule is present in TZ, the rules specified by the
tzfile(5)-format file posixrules in the system time conversion
information directory are used, with the standard and summer time
offsets from UTC replaced by those specified by the offset values in
TZ.
For compatibility with System V Release 3.1, a semicolon (;) may be
used to separate the rule from the rest of the specification.
If the TZ environment variable does not specify a tzfile(5)-format and
cannot be interpreted as a direct specification, UTC is used.
FILES
/usr/local/etc/zoneinfo time zone information directory
/usr/local/etc/zoneinfo/localtime local time zone file
/usr/local/etc/zoneinfo/posixrules used with POSIX-style TZ's
/usr/local/etc/zoneinfo/GMT for UTC leap seconds
If /usr/local/etc/zoneinfo/GMT is absent, UTC leap seconds are loaded
from /usr/local/etc/zoneinfo/posixrules.
SEE ALSO
getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)
NEWTZSET(3)