NAME
tzset, tzsetwall
— initialize time conversion
information
SYNOPSIS
#include
<time.h>
void
tzset(void);
void
tzsetwall(void);
DESCRIPTION
tzset()
uses the value of the environment variable TZ to set
time conversion information used by
localtime(3). 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
by localtime(3).
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, it is used as a pathname of a file from which to
read the time conversion information.
If TZ appears in the environment and its
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 (without whitespace between std and
offset):
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 (‘:’), digits, comma (‘,’), minus (‘-’), plus (‘+’), and ASCII NUL are allowed.
- offset
- Indicates the value one must add to the local time to arrive at
Coordinated Universal Time. 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.
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:
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- Day d (1 <= d <= 6) of week n (1 <= n <= 5) of month m (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 dth 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
TZ, 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 (‘;’) may be used to separate the rule from the rest of the specification.
If the TZ environment variable does not
specify a tzfile(5) format file and cannot be interpreted as a direct
specification, UTC is used.
tzsetwall()
behaves identically to tzset() but it only uses the
/etc/localtime file (that is, it ignores the
TZ environment variable).
FILES
- /usr/share/zoneinfo
- time zone information directory
- /etc/localtime
- local time zone file
- /usr/share/zoneinfo/posixrules
- used with POSIX-style
TZs - /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
STANDARDS
The tzset() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”). The
tzsetwall() function is an extension to that
specification.