newtzset(3) Library Functions Manual newtzset(3)

tzset - initialize time conversion information

#include <time.h>
timezone_t tzalloc(char const *TZ);
void tzfree(timezone_t tz);
void tzset(void);
/∗ Optional and obsolescent:  ∗/
extern char *tzname[];
extern long timezone;
extern int daylight;
cc ... -ltz

The tzalloc function allocates and returns a timezone object described by TZ.

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 “/” 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:

stdoffset[dst[offset][,rule]]

Where:

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.
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 “+”.
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. 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:
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.
The zero-based Julian day (0 ≤ n ≤ 365). Leap days are counted, and it is possible to refer to February 29.
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 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 timezone.

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”.
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 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.

As a side effect, the tzset function sets some external variables if the platform defines them. It sets tzname[0] and tzname[1] to pointers to strings that are time zone abbreviations to be used with standard and daylight saving time, respectively. It also sets timezone to be the number of seconds that standard time is west of the Prime Meridian, and daylight to be zero if daylight saving time is never in effect, non-zero otherwise.

If successful, the tzalloc function returns a nonnull pointer to the newly allocated object. Otherwise, it returns a null pointer and sets errno.

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).

/etc/localtime local timezone file
/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.

getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5).

Portable code should not rely on the contents of the external variables tzname, timezone and daylight as their contents are unspecified (and do not make sense in general) when a geographical TZ is used. In multithreaded applications behavior is undefined if one thread accesses one of these variables while another thread invokes tzset. A future version of POSIX is planned to remove these variables; callers can instead use the tm_gmtoff and tm_zone members of struct tm, or use strftime with "%z" or "%Z".

Time Zone Database