TZSET(3)

NAME
tzset, tzalloc, tzgetname, tzgetgmtoff, tzfree - initialize time conversion information

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
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 zone 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):

    stdoffset[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) timezone. 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 (,), 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:

        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 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 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 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
    default DST rules (obsolete, and can cause bugs if present) used with POSIX-style TZ
/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 p1003.1-88.

BUGS
Neither the tzgetname() nor tzgmtoff() functions have the ability to specify the point in time for which the requested data should be returned.