ZIC(8) System Manager's Manual ZIC(8)

zic — time zone compiler

zic [-v] [-d directory] [-L leapsecondfilename] [-l timezone] [-p timezone] [filename ...]

zic reads text from the file(s) named on the command line and creates the time conversion information files specified in this input. If a filename is “-”, the standard input is read.

These options are available:

directory
Create time conversion information files in the named directory rather than in the standard directory named below.
leapsecondfilename
Read leap second information from the file with the given name. If this option is not used, no leap second information appears in output files.
timezone
Use the given time zone as local time. zic will act as if the input contained a link line of the form

Link timezone localtime
timezone
Use the given time zone's rules when handling POSIX-format time zone environment variables. zic will act as if the input contained a link line of the form

Link timezone posixrules
Be more verbose, and complain about the following situations:

The input data specifies a link to a link.

A year that appears in a data file is outside the range of years representable by time(3) values.

A time of 24:00 or more appears in the input. Pre-1998 versions of zic prohibit 24:00, and pre-2007 versions prohibit times greater than 24:00.

A rule goes past the start or end of the month. Pre-2004 versions of zic prohibit this.

The output file does not contain all the information about the long-term future of a zone, because the future cannot be summarized as an extended POSIX TZ string. For example, as of 2013 this problem occurs for Iran's daylight-saving rules for the predicted future, as these rules are based on the Iranian calendar, which cannot be represented.

The output contains data that may not be handled properly by client code designed for older zic output formats. These compatibility issues affect only time stamps before 1970 or after the start of 2038.

A time zone abbreviation has fewer than 3 characters. POSIX requires at least 3.

Input lines are made up of fields. Fields are separated from one another by one or more whitespace characters. Leading and trailing whitespace on input lines is ignored. An unquoted sharp character (#) in the input introduces a comment which extends to the end of the line the sharp character appears on. White space characters and sharp characters may be enclosed in double quotes (") if they're to be used as part of a field. Any line that is blank (after comment stripping) is ignored. Non-blank lines are expected to be of one of three types: rule lines, zone lines, and link lines.

Names (such as month names) must be in English and are case insensitive. Abbreviations, if used, must be unambiguous in context.

A rule line has the form:

Rule  NAME  FROM  TO    TYPE  IN    ON       AT    SAVE  LETTER/S

For example:

Rule  US    1967  1973  -     Apr   lastSun  2:00  1:00  D

The fields that make up a rule line are:

Gives the (arbitrary) name of the set of rules this rule is part of.
Gives the first year in which the rule applies. Any integer year can be supplied; the Gregorian calendar is assumed. The word minimum (or an abbreviation) means the minimum year representable as an integer. The word maximum (or an abbreviation) means the maximum year representable as an integer. Rules can describe times that are not representable as time values, with the unrepresentable times ignored; this allows rules to be portable among hosts with differing time value types.
Gives the final year in which the rule applies. In addition to minimum and maximum (as above), the word (or an abbreviation) may be used to repeat the value of the FROM field.
Gives the type of year in which the rule applies. This field is obsolete and should always be “-”.
Names the month in which the rule takes effect. Month names may be abbreviated.
Gives the day on which the rule takes effect. Recognized forms include:

5
the fifth of the month
lastSun
the last Sunday in the month
lastMon
the last Monday in the month
Sun>=8
first Sunday on or after the eighth
Sun<=25
last Sunday on or before the 25th

Names of days of the week may be abbreviated or spelled out in full. Note that there must be no spaces within the ON field.

Gives the time of day at which the rule takes effect. Recognized forms include:

2
time in hours
2:00
time in hours and minutes
15:00
24-hour format time (for times after noon)
1:28:14
time in hours, minutes, and seconds
-
equivalent to 0

where hour 0 is midnight at the start of the day, and hour 24 is midnight at the end of the day. Any of these forms may be followed by the letter w if the given time is local “wall clock” time, s if the given time is local “standard” time, or (or or ) if the given time is universal time; in the absence of an indicator, wall clock time is assumed.

Gives the amount of time to be added to local standard time when the rule is in effect. This field has the same format as the AT field (although, of course, the w and s suffixes are not used).
Gives the “variable part” (for example, the “S” or “D” in “EST” or “EDT”) of time zone abbreviations to be used when this rule is in effect. If this field is “-”, the variable part is null.

A zone line has the form:

Zone NAME GMTOFF RULES/SAVE FORMAT [UNTILYEAR [MONTH [DAY [TIME]]]]

For example:

Zone Australia/Adelaide 9:30 Aus CST 1971 Oct 31 2:00

The fields that make up a zone line are:

The name of the time zone. This is the name used in creating the time conversion information file for the zone.
The amount of time to add to UT to get standard time in this zone. This field has the same format as the AT and SAVE fields of rule lines; begin the field with a minus sign if time must be subtracted from UT.
The name of the rule(s) that apply in the time zone or, alternately, an amount of time to add to local standard time. If this field is “-” then standard time always applies in the time zone.
The format for time zone abbreviations in this time zone. The pair of characters is used to show where the “variable part” of the time zone abbreviation goes. Alternately, a format can use the pair of characters to stand for the UTC offset in the form ±hh, ±, or ±, using the shortest form that does not lose information, where hh, , and are the hours, minutes, and seconds east (+) or west (−) of UTC. Alternately, a slash (/) separates standard and daylight abbreviations.
The time at which the UT offset or the rule(s) change for a location. It is specified as a year, a month, a day, and a time of day. If this is specified, the time zone information is generated from the given UT offset and rule change until the time specified. The month, day, and time of day have the same format as the IN, ON, and AT fields of a rule; trailing fields can be omitted, and default to the earliest possible value for the missing fields.

The next line must be a “continuation” line; this has the same form as a zone line except that the string “Zone” and the name are omitted, as the continuation line will place information starting at the time specified as the “until” information in the previous line in the file used by the previous line. Continuation lines may contain “until” information, just as zone lines do, indicating that the next line is a further continuation.

A link line has the form:

Link	LINK-FROM	LINK-TO

For example:

Link	Europe/Istanbul	Asia/Istanbul

Except for continuation lines, lines may appear in any order in the input. However, the behavior is unspecified if multiple zone or link lines define the same name, or if the source of one link line is the target of another.

Lines in the file that describes leap seconds have the following form:

Leap	YEAR	MONTH	DAY	HH:MM:SS	CORR	R/S

For example:

Leap	1974	Dec	31	23:59:60	+	S

The , , , and fields tell when the leap second happened. The field should be “+” if a second was added or “-” if a second was skipped. The field should be (an abbreviation of) “Stationary” if the leap second time given by the other fields should be interpreted as UTC or (an abbreviation of) “Rolling” if the leap second time given by the other fields should be interpreted as local wall clock time.

Here is an extended example of zic input, intended to illustrate many of its features.

# Rule	NAME	FROM	TO	TYPE	IN	ON	AT	SAVE	LETTER/S
Rule	Swiss	1941	1942	-	May	Mon>=1	1:00	1:00	S
Rule	Swiss	1941	1942	-	Oct	Mon>=1	2:00	0	-

Rule	EU	1977	1980	-	Apr	Sun>=1	1:00u	1:00	S
Rule	EU	1977	only	-	Sep	lastSun	1:00u	0	-
Rule	EU	1978	only	-	Oct	 1	1:00u	0	-
Rule	EU	1979	1995	-	Sep	lastSun	1:00u	0	-
Rule	EU	1981	max	-	Mar	lastSun	1:00u	1:00	S
Rule	EU	1996	max	-	Oct	lastSun	1:00u	0	-

# Zone	NAME		GMTOFF	RULES/SAVE	FORMAT	UNTIL
Zone	Europe/Zurich	0:34:08	-		LMT	1853 Jul 16
			0:29:46	-		BMT	1894 Jun
			1:00	Swiss		CE%sT	1981
			1:00	EU		CE%sT

Link	Europe/Zurich	Switzerland

In this example, the zone is named Europe/Zurich but it has an alias as Switzerland. This example says that Zurich was 34 minutes and 8 seconds west of UT until 1853-07-16 at 00:00, when the legal offset was changed to 7°26′22.50″; although this works out to 0:29:45.50, the input format cannot represent fractional seconds so it is rounded here. After 1894-06-01 at 00:00 Swiss daylight saving rules (defined with lines beginning with "Rule Swiss") apply, and the UT offset became one hour. From 1981 to the present, EU daylight saving rules have applied, and the UTC offset has remained at one hour.

In 1940, daylight saving time applied from November 2 at 00:00 to December 31 at 00:00. In 1941 and 1942, daylight saving time applied from the first Monday in May at 01:00 to the first Monday in October at 02:00. The pre-1981 EU daylight-saving rules have no effect here, but are included for completeness. Since 1981, daylight saving has begun on the last Sunday in March at 01:00 UTC. Until 1995 it ended the last Sunday in September at 01:00 UTC, but this changed to the last Sunday in October starting in 1996.

For purposes of display, "LMT" and "BMT" were initially used, respectively. Since Swiss rules and later EU rules were applied, the display name for the timezone has been CET for standard time and CEST for daylight saving time.

/etc/localtime
link to local time zone
/usr/share/zoneinfo
standard directory used for created files

ctime(3), tzfile(5), zdump(8)

For areas with more than two types of local time, you may need to use local standard time in the AT field of the earliest transition time's rule to ensure that the earliest transition time recorded in the compiled file is correct.

If, for a particular zone, a clock advance caused by the start of daylight saving coincides with and is equal to a clock retreat caused by a change in UT offset, zic produces a single transition to daylight saving at the new UT offset (without any change in wall clock time). To get separate transitions use multiple zone continuation lines specifying transition instants using universal time.

June 23, 2025 Debian