NAME
tzset
, tzalloc
,
tzgetname
, tzgetgmtoff
,
tzfree
—
initialize time conversion
information
LIBRARY
library “libc”
SYNOPSIS
#include
<time.h>
timezone_t
tzalloc
(const
char *zone);
void
tzfree
(timezone_t
restrict tz);
const char *
tzgetname
(timezone_t
restrict tz, int
isdst);
long
tzgetgmtoff
(timezone_t
restrict tz, int
isdst);
void
tzset
(void);
DESCRIPTION
Thetzalloc
()
function takes as an argument a timezone name and returns a
timezone_t object suitable to be used in the
ctime_rz
(),
localtime_rz
(),
and
mktime_z
()
functions.
If tz is not a valid timezone
description, or if the object cannot be allocated,
tzalloc
()
returns a NULL
pointer and sets
errno.
A NULL
pointer may be
passed to
tzalloc
()
instead of a timezone name, to refer to the current system timezone. An
empty timezone string indicates Coordinated Universal Time (UTC).
Note that instead of setting the environment
variable TZ, and globally changing the behavior of the
calling program, one can use multiple timezones at the same time by using
separate timezone_t objects allocated by
tzalloc
()
and calling the “z” variants of the functions. The
tzfree
()
function deallocates tz, which was previously
allocated by tzalloc
(). This invalidates any
tm_zone pointers that tz was
used to set. The function
tzgetname
()
returns the name for the given tz. If
isdst is 0, the call is
equivalent to tzname[0]. If
isdst is set to 1 the call is
equivalent to tzname[1]. The return values for both
tzgetname
() and
tzgmtoff
()
correspond to the latest time for which data is available, even if that
refers to a future time. Finally, the
tzgetgmtoff
()
function acts like tzgetname
() only it returns the
offset in seconds from GMT for the timezone. If there is no match, then
-1
is returned and errno is
set to ESRCH
. The
tzset
()
function acts like tzalloc(getenv("TZ"))
,
except it saves any resulting timezone object into internal storage that is
accessed by
localtime
(),
localtime_r
(),
and
mktime
().
The anonymous shared timezone 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 /etc/localtime is used
by localtime(3). If TZ
appears in the
environment but its value is the empty string, UT is used, with the
abbreviation “UTC” and without leap second correction; please
see ctime(3). 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 /usr/share/zoneinfo.
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):
std
offset
[dst
[offset
][,rule
]]
where:
std
anddst
- Three or more bytes that are the designation for the standard
(
std
) or the alternative (dst
such as daylight saving time) timezone. Onlystd
is required; ifdst
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 (,), minus (-), 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>
andNUL
. 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. Theoffset
followingstd
is required. If nooffset
followsdst
, 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 timezone 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 seconddate
describes when the change back happens. Eachtime
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:J
n- 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.
M
m.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.
time
has the same format asoffset
except that POSIX does not allow a leading sign “-” or “+” is allowed. As an extension to POSIX, the hours part oftime
can range from -167 through 167; this allows for unusual rules such as “the Saturday before the first Sunday of March”. The default, iftime
is not given, is02:00:00
.
Here are some examples of TZ values that directly specify the timezone rules; they use some of the extensions to POSIX.
- EST5
- stands for US Eastern Standard Time (EST), 5 hours behind UT, without daylight saving.
- FJT-12FJST,M11.1.0,M1.3.4/75
- <+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
/usr/share/zoneinfo 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
- /etc/localtime
- local timezone file
- /usr/share/zoneinfo
- local timezone information directory
- /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
STANDARDS
The tzset
() function conforms to
IEEE Std 1003.1-1988 (“POSIX.1”).
BUGS
Neither the tzgetname
() nor
tzgmtoff
() functions have the ability to specify the
point in time for which the requested data should be returned.