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
If TZ is a null pointer, tzalloc uses 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.
If TZ is the empty string, tzalloc uses Universal Time (UT), 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 contents are used as a pathname, a pathname beginning with .q "/" is used as-is; otherwise the pathname is 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:
std\|offset[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]]
rule Indicates when to change to and back from daylight saving time. The rule has the form:
date/time,date/time
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.1-2017.
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 TZ specifies daylight saving time but does not specify a rule , and the optional tzfile (5)-format file posixrules is present in the system time conversion information directory, the rules in posixrules are used, with the posixrules standard and daylight saving time offsets from UT replaced by those specified by the offset values in TZ . However, the posixrules file is obsolete: if it is present it is only for backward compatibility, and it does not work reliably. Therefore, if a TZ string directly specifies a timezone with daylight saving time, it should specify the daylight saving rules explicitly.
For compatibility with System V Release 3.1, a semicolon ( ; ) may be used to separate the rule from the rest of the specification; this is an extension to POSIX.
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 getenv fails, tzset acts like tzalloc(nullptr) ; if the implied call to tzalloc fails, tzset falls back on UT.
EOVERFLOW TZ directly specifies time conversion information, and contains an integer out of machine range or a time zone abbreviation that is too long for this platform.
The tzalloc function may also fail and set errno for any of the errors specified for the routines access (2), close (2), malloc (3), open (2), and read (2).
/usr/share/zoneinfo timezone directory
/usr/share/zoneinfo/posixrules default DST rules (obsolete)
/usr/share/zoneinfo/GMT for UTC leap seconds
If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/share/zoneinfo/GMT0 if present.