tzset(3) manual page

The tzset() function initializes time conversion information used by the library routine localtime(3). The environment variable TZ specifies how this is done.

If TZ does not appear in the environment, the best available approximation to local wall clock time, as specified by the tzfile(5)-format file /etc/localtime is used.

If TZ appears in the environment but its value is a null string, Coordinated Universal Time ( UTC) is used (without leap second correction).

If TZ appears in the environment and its value begins with a colon (‘amp;:’), the rest of its value is used as a pathname of a tzfile(5)-format file from which to read the time conversion information. If the first character of the pathname is a slash (‘/’) it is used as an absolute pathname; otherwise, it is used as a pathname relative to the system time conversion information directory.

If its value does not begin with a colon, it is first used as the pathname of a file (as described above) from which to read the time conversion information. If that file cannot be read, the value is then interpreted as a direct specification (the format is described below) of the time conversion information.

If the TZ environment variable does not specify a tzfile(5)-format file and cannot be interpreted as a direct specification, UTC is used.

The tzsetwall() function sets things up so that localtime(3) returns the best available approximation of local wall clock time.

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:

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 (‘amp;:’), digits, comma (‘amp;,’), minus (‘-’), plus (‘+’), and ASCII NUL are allowed.

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, 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: date/time,date/time
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.
The format of date is one of the following:

J n 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.

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 " 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 no leading sign (‘-’) or (‘+’) is allowed. The default, if time is not given, is 02:00:00.

If no rule is present in the TZ specification, the rules specified by the tzfile(5)-format file posixrules in the system time conversion information directory 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 (‘amp;;’) may be used to separate the rule from the rest of the specification.

date(1), gettimeofday(2), ctime(3), getenv(3), time(3), tzfile(5)

`/etc/localtime`	local time zone file
`/usr/share/zoneinfo`	time zone directory
`/usr/share/zoneinfo/posixrules`
	rules for POSIX-style TZ's
`/usr/share/zoneinfo/Etc/GMT`	for UTC leap seconds

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Manual Pages — TZSET

NAME

CONTENTS

LIBRARY

SYNOPSIS

DESCRIPTION

SPECIFICATION FORMAT

FILES

SEE ALSO

HISTORY

J n	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.
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 " 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 no leading sign (‘`-`’) or (‘`+`’) is allowed. The default, if time is not given, is 02:00:00.