TZSET(3) BSD Programmer's Manual TZSET(3)NAME
tzset - initialize time conversion information
SYNOPSIS
#include <time.h>
void
tzset(void);
DESCRIPTIONtzset() uses the value of the environment variable TZ to set time conver-
sion information used by localtime(3). If TZ does not appear in the en-
vironment, 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(3).
If TZ appears in the environment but its value is a null string, Coordi-
nated Universal Time (UTC) is used.
As a side effect, calling tzset() also initialises the leap second table
for tai2utc(), utc2tai(), tai_leaps(), and tai_isleap().
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 in-
formation.
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 infor-
mation, 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 stan-
dard (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 (spaces
inserted for clarity):
hh [: mm [: ss]]
The minutes (mm) and seconds (ss) are optional. The hour
(hh) is required and may be a single digit. The offset fol-
lowing 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 (spaces added for clarity):
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 (spaces added for
clarity):
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 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 lead-
ing 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.
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/UTC for UTC leap seconds
If /usr/share/zoneinfo/UTC is absent, UTC leap seconds are loaded from
/usr/share/zoneinfo/posixrules.
SEE ALSOctime(3), getenv(3), strftime(3), tai_leaps(3), time(3), tzfile(5)MirOS BSD #10-current May 24, 1999 1
