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NEWTZSET(3) Library Functions Manual NEWTZSET(3)
NAME
tzset - initialize time conversion information
SYNOPSIS
#include <time.h>
timezone_t tzalloc(char const *TZ);
void tzfree(timezone_t tz);
void tzset(void);
cc ... -ltz
DESCRIPTION
Tzalloc allocates and returns a time zone object described by TZ. If
TZ is not a valid time zone description, or if the object cannot be
allocated, tzalloc returns a null pointer and sets errno.
Tzfree frees a time zone object tz, which should have been successfully
allocated by tzalloc. This invalidates any tm_zone pointers that tz
was used to set.
Tzset acts like tzalloc(getenv("TZ")), except it saves any resulting
time zone object into internal storage that is accessed by localtime,
localtime_r, and mktime. The anonymous shared time zone object is
freed by the next call to tzset. If the implied call to tzalloc fails,
tzset falls back on Universal Time (UT).
If TZ is null, 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. If TZ is the empty
string, 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 is nonnull and nonempty:
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 the alternative (dst, such
as daylight saving time) time zone. Only std is
required; if dst is missing, then daylight saving
time does not apply in this locale. Upper- and
lowercase letters are explicitly allowed. Any
characters except a leading colon (:), digits,
comma (,), ASCII minus (-), ASCII plus (+), and
NUL bytes are allowed. Alternatively, a
designation can be surrounded by angle brackets <
and >; in this case, the designation can contain
any characters other than > and NUL.
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, daylight saving 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
daylight saving time. The rule has the form:
date/time,date/time
where the first date describes when the change
from standard to daylight saving 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 UT,
without daylight saving.
<+12>-12<+13>,M11.1.0,M1.2.1/147
stands for Fiji time, 12 hours ahead of UT, springing forward on
November's first Sunday at 02:00, and falling back on January's
second Monday at 147:00 (i.e., 03:00 on the first Sunday on or
after January 14). The abbreviations for standard and daylight
saving time are "+12" and "+13".
IST-2IDT,M3.4.4/26,M10.5.0
stands for Israel Standard Time (IST) and Israel Daylight Time
(IDT), 2 hours ahead of UT, 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.
<-04>4<-03>,J1/0,J365/25
stands for permanent daylight saving time, 3 hours behind UT
with abbreviation "-03". 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 <-04> is a placeholder.
<-03>3<-02>,M3.5.0/-2,M10.5.0/-1
stands for time in western Greenland, 3 hours behind UT, where
clocks follow the EU rules of springing forward on March's last
Sunday at 01:00 UT (-02:00 local time, i.e., 22:00 the previous
day) and falling back on October's last Sunday at 01:00 UT
(-01:00 local time, i.e., 23:00 the previous day). The
abbreviations for standard and daylight saving time are "-03"
and "-02".
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 daylight saving
time offsets from UT 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.
FILES
/usr/share/zoneinfo time zone information directory
/usr/share/zoneinfo/localtime local time zone file
/usr/share/zoneinfo/posixrules used with POSIX-style TZ's
/usr/share/zoneinfo/GMT for UTC leap seconds
If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
/usr/share/zoneinfo/posixrules.
SEE ALSO
getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)
NEWTZSET(3)
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