TIMEZONE(4) CLIX TIMEZONE(4)
NAME
TIMEZONE - Default system time zone file
DESCRIPTION
The /etc/TIMEZONE file sets and exports the time zone environmental
variable TZ.
This file is ``dotted'' into other files that must know the time zone.
In most installations, TZ is set to the correct value by default when the
user logs on, with the local /etc/profile file (see profile(4)).
The form of the TZ variable is as follows, where std indicates standard
time, dst indicates daylight savings time, and offset indicates the time
that must be added to local time to arrive at Greenwich Mean Time (GMT):
CLIX: stdoffset[dst[offset][; clix_rule]]
POSIX: stdoffset[dst[offset][, posix_rule]]
In the POSIX-compliant version, note the use of the comma (,) instead of
the semicolon (;) to separate the rule from the rest of the construct.
The std and dst specifiy the standard time zone and the alternate time
zone (such as Daylight Savings Time), respectively. The time zone name
contains three alphabetic characters. Only std is required; if dst is not
given, the alternate time zone does not apply to the locale. Uppercase
and lowercase letters are permitted in the std and dst time zone names.
The offset has the following form, where hh is the number of hours
(between 0 and 24), mm is the number of minutes (between 0 and 59), and ss
is the number of seconds (between 0 and 59):
[+|-]hh[mm[ss]]
A leading - indicates that the time is east of the Prime Meridian; a
leading + indicates that the time is west of the Prime Meridian.
The *_rule describes when to change to daylight savings time, and when to
change back to standard time. If no rule is specified, the days for the
United States for that year are used, and the time defaults to 02:00:00.
The CLIX rule, POSIX-compliant rule, and the United States rule are
explained in the following sections.
Using the CLIX Rule
The clix_rule has the following form, where start_date and start_time
describe when to change to daylight savings time, and end_date and
end_time describe when to change back to standard time:
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start_date/start_time,end_date/end_time
The *_date fields have a value of n, which is the Julian day and has
values from 1 to 366 inclusive.
The format of the *_time fields is the same as for offset, except that no
leading sign is allowed. If no time is given, the default is 00:00:00.
Using the POSIX Rule
The posix_rule has the following form, where start_date and start_time
describe when to change to daylight savings time, and end_date and
end_time describe when to change back to standard time:
start_date/start_time,end_date/end_time
The format of the *_date fields is one of the following:
Jn The Julian day n (between 1 and 365). Leap days are not counted;
that is, February 28 is always day 59 and March 1 is day 60. It is
not possible to refer to February 29.
n In the POSIX-compliant version, the zero-based Julian day (between
0 and 365). Leap days are counted, and it is possible to refer to
February 29.
Mm.n.d In the POSIX-compliant version, the dth day (between 0 and 6) of
the nth week (between 0 and 5) of the mth month (between 0 and 12).
If m is 5, it means "the last day d in month m," which may occur in
either the fourth or fifth week. Week 1 is the first week in which
day d occurs; day 0 is Sunday.
The format of the *_time fields is the same as for offset, except that no
leading sign is allowed. If no time is given, the default is 02:00:00.
Using the United States Rule
If the alternate time zone start and end dates are not provided, the days
for the United States that year will be used. The following table shows
the days that are used, according to the year:
From 1987 to the present
The start_date is the first Sunday in April, and the end_date is
the last Sunday in October.
From 1976 to 1986
The start_date is the last Sunday in April, and the end_date is
last Sunday in October.
1975 The start_date is the last Sunday in February, and the end_date
is last Sunday in October.
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1974 The start_date is the first Sunday in January, and the end_date
is last Sunday in November.
From 1970 to 1973
The start_date is last Sunday in April, and the end_date is the
last Sunday in October.
If the time is not provided, it defaults to 02:00:00.
EXAMPLES
1. A simple setting for New Jersey could be as follows:
TZ=EST5EDT
In this example, EST is the abbreviation for the main time zone, 5 is
the difference, in hours, between Greenwich Mean Time and the main
time zone, and EDT is the abbreviation for the alternate time zone.
The contents of /etc/TIMEZONE for this setting would be as follows:
# Time Zone
TZ=EST5EDT
export TZ
2. The most complex representation of the same setting, for the year
1986, is as follows:
TZ="EST5:00:00EDT4:00:00;117/2:00:00,299/2:00:00"
In this example:
⊕ EST is the abbreviation for the main time zone.
⊕ 5:00:00 is the difference, in hours, minutes, and seconds, between
GMT and the main time zone.
⊕ EDT is the abbreviation for the alternate time zone.
⊕ 4:00:00 is the difference, in hours, minutes, and seconds, between
GMT and the alternate time zone.
⊕ 117 is the number of the day of the year (Julian day) when the
alternate time zone will take effect.
⊕ 2:00:00 is the number of hours, minutes, and seconds past midnight
when the alternate time zone will take effect.
⊕ 299 is the number of the day of the year when the alternate time
zone will end.
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⊕ 2:00:00 is the number of hours, minutes, and seconds past midnight
when the alternate time zone will end.
3. A southern hemisphere setting, such as for the Cook Islands, could be
as follows:
TZ="KDT9:30KST10:00;64/5:00,303/20:00"
In this example:
⊕ KDT is the abbreviation for the main time zone, which is 9 hours
and 30 minutes later than GMT.
⊕ KST is the abbreviation for the alternate time zone, which is 10
hours later than GMT.
⊕ The starting date of daylight savings time is the 64th day at 5:00
A.M.
⊕ The ending date of daylight savings time is the 303rd day at 8:00
P.M.
4. In this POSIX-compliant setting, daylight savings time occurs between
February 27 and March 2. February 29 in leap years does not affect
the time change on March 2.
TZ="std0dst,J58,J61"
5. In this POSIX-compliant setting, daylight savings time occurs between
the first Tuesday in January and the fifth Thursday in February. If
there is no fifth week, the last week of the month is used.
TZ="std0dst,M01.1.2,M02.5.5"
NOTES
When the clix_rule is used, the TZ variable must be surrounded by double
quotes as shown.
The system administrator must change the Julian start and end days at the
beginning of a leap year and at the beginning of the following year if the
n format is used for the date in the clix_rule and the posix_rule of the
TZ variable is used.
Setting the time during the interval of change from the main time zone to
the alternate time zone, or vice versa, can produce unpredictable results.
RELATED INFORMATION
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Commands: rc2(1)
Functions: ctime(3)
Files: profile(4), environ(4)
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