NAME
tzset,
tzalloc,
tzgetname,
tzgetgmtoff,
tzfree —
initialize time conversion information
LIBRARY
Standard C Library (libc, -lc)
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
The
tzalloc() 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 time zone 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]. 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 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
UTC. 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, Universal Time (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):
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 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:
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:
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
“+” is allowed. 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,M11.1.0,M1.3.4/75
- stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12
hours ahead of UTC, springing forward on November's first Sunday at 02:00,
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
/usr/share/zoneinfo 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.
FILES
- /etc/localtime
- local time zone file
- /usr/share/zoneinfo
- time zone 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
ctime(3),
getenv(3),
strftime(3),
time(3),
tzfile(5)
STANDARDS
The
tzset() function conforms to
IEEE Std
1003.1-1988 (“POSIX.1”).