2009-05-17 by Arthur David Olson.
#include <time.h>timezone_t tzalloc(char const *TZ);
void tzfree(timezone_t tz);
void tzset(void);
cc ... \*-ltz
The tzfree function frees a timezone object tz , which should have been successfully allocated by tzalloc . This invalidates any tm_zone pointers that tz was used to set.
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 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):
std\|offset[dst[offset][,rule]]Where:
15 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]]
rule Indicates when to change to and back from daylight saving time. The rule has the form:
date/time,date/time
10 J n The Julian day n "(1 \(<=" " n " "\(<= 365). Leap days are not counted; that is, in all years \*(en including leap years \*(en 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 .q "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.
Here are some examples of TZ values that directly specify the timezone; 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 .q "+12" and .q "+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 .q "\*-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 .q "\*-03" and .q "\*-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.
/usr/share/zoneinfo/localtime local timezone file
/usr/share/zoneinfo/posixrules default DST rules (obsolete, and can cause bugs if present)
/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 .