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newtzset.3.txt
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newtzset.3.txt
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NAME
tzset - initialize time conversion information
SYNOPSIS
void tzset()
cc ... -ltz
DESCRIPTION
Tzset uses the value of the environment variable TZ to set
time conversion information used by localtime. 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 localtime in the system time
conversion information directory, is used by localtime. 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 is not a null string:
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 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
(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
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. 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:
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 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 and cannot be interpreted as a direct
specification, UTC is used.
FILES
/usr/local/etc/zoneinfo time zone information
directory
/usr/local/etc/zoneinfo/localtime local time zone file
/usr/local/etc/zoneinfo/posixrules used with POSIX-style
TZ's
/usr/local/etc/zoneinfo/GMT for UTC leap seconds
If /usr/local/etc/zoneinfo/GMT is absent, UTC leap seconds
are loaded from /usr/local/etc/zoneinfo/posixrules.
SEE ALSO
getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)