zic - timezone compiler
... ] [ filename
program reads text from the file(s) named on the command line and
creates the time conversion information files specified in this input. If a
is “-”, standard input is read.
- Output version information and exit.
- Output short usage message and exit.
- -d directory
- Create time conversion information files in the named directory rather
than in the standard directory named below.
- -l timezone
- Use timezone as local time. zic will act as if the input
contained a link line of the form
Link timezone localtime
- -p timezone
- Use timezone's rules when handling POSIX-format timezone
environment variables. zic will act as if the input contained a
link line of the form
Link timezone posixrules
- -L 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
- Be more verbose, and complain about the following situations:
The input specifies a link to a link.
A year that appears in a data file is outside the range of years representable
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
The output file does not contain all the information about the long-term future
of a timezone, 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 timestamps before 1970 or after the start of 2038.
A time zone abbreviation has fewer than 3 characters. POSIX requires at least 3.
An output file name contains a byte that is not an ASCII letter,
“-”, “/”, or “_”; or it contains a
file name component that contains more than 14 bytes or that starts with
- Limit time values stored in output files to values that are the same
whether they're taken to be signed or unsigned. You can use this option to
generate SVVS-compatible files.
Input files should be text files, that is, they should be a series of zero or
more lines, each ending in a newline byte and containing at most 511 bytes,
and without any NUL bytes. The input text's encoding is typically UTF-8 or
ASCII; it should have a unibyte representation for the POSIX Portable
Character Set (PPCS)
and the encoding's non-unibyte characters should consist entirely of non-PPCS
bytes. Non-PPCS characters typically occur only in comments: although output
file names and time zone abbreviations can contain nearly any character, other
software will work better if these are limited to the restricted syntax
described under the -v
Input lines are made up of fields. Fields are separated from one another by one
or more white space characters. The white space characters are space, form
feed, carriage return, newline, tab, and vertical tab. Leading and trailing
white space 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. Nonblank lines
are expected to be of one of three types: rule lines, zone lines, and link
Names must be in English and are case insensitive. They appear in several
contexts, and include month and weekday names and keywords such as
, and Zone
. A name can be
abbreviated by omitting all but an initial prefix; any abbreviation must be
unambiguous in context.
A rule line has the form
Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
Rule US 1967 1973 - Apr lastSun 2:00w 1:00 D
The fields that make up a rule line are:
- Gives the name of the rule set that contains this line. The name must
start with a character that is neither an ASCII digit nor
“-” nor “+”. To allow for future extensions,
an unquoted name should not contain characters from the set
- Gives the first year in which the rule applies. Any signed integer year
can be supplied; the proleptic Gregorian calendar is assumed, with year 0
preceding year 1. The word minimum (or an abbreviation) means the
indefinite past. The word maximum (or an abbreviation) means the
indefinite future. 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 only (or an
abbreviation) may be used to repeat the value of the FROM
- should be “-” and is present for compatibility with older
versions of zic in which it could contain year types.
- Names the month in which the rule takes effect. Month names may be
- 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
A weekday name (e.g., Sunday) or a weekday name preceded by
“last” (e.g., lastSunday) may be abbreviated or
spelled out in full. Note that there must be no spaces within the
- Gives the time of day at which the rule takes effect. Recognized forms
2 time in hours
2:00 time in hours and minutes
01:28:14 time in hours, minutes, and seconds
15:00 24-hour format time (for times after noon)
260:00 260 hours after 00:00
-2:30 2.5 hours before 00:00
- 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
u (or g or z) if the given time is universal time; in
the absence of an indicator, wall clock time is assumed. The intent is
that a rule line describes the instants when a clock/calendar set to the
type of time specified in the AT field would show the specified
date and time of day.
- 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).
Negative offsets are allowed; in Ireland, for example, daylight saving
time is observed in winter and has a negative offset relative to Irish
Standard Time. The offset is merely added to standard time; for example,
zic does not distinguish a 10:30 standard time plus an 0:30
SAVE from a 10:00 standard time plus a 1:00 SAVE.
- 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
A zone line has the form
Zone NAME UTOFF RULES FORMAT [UNTIL]
Zone Asia/Amman 2:00 Jordan EE%sT 2017 Oct 27 01:00
The fields that make up a zone line are:
- The name of the timezone. This is the name used in creating the time
conversion information file for the timezone. It should not contain a file
name component “.” or “..”; a file name
component is a maximal substring that does not contain
- The amount of time to add to UT to get standard time. 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 rules that apply in the timezone or, alternatively, a
field in the same format as a rule-line SAVE column, giving of the amount
of time to be added to local standard time effect, and whether the
resulting time is standard or daylight saving. If this field is -
then standard time always applies. When an amount of time is given, only
the sum of standard time and this amount matters.
- The format for time zone abbreviations. The pair of characters %s
is used to show where the “variable part” of the time zone
abbreviation goes. Alternatively, a format can use the pair of characters
%z to stand for the UT offset in the form ±hh,
±hhmm, or ±hhmmss, using the shortest form
that does not lose information, where hh, mm, and ss
are the hours, minutes, and seconds east (+) or west (−) of UT.
Alternatively, a slash (/) separates standard and daylight abbreviations.
To conform to POSIX, a time zone abbreviation should contain only
alphanumeric ASCII characters, “+” and
- The time at which the UT offset or the rule(s) change for a location. It
takes the form of YEAR [MONTH [DAY [TIME]]]. If this is specified, the
time zone information is generated from the given UT offset and rule
change until the time specified, which is interpreted using the rules in
effect just before the transition. 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
- 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.
If a zone changes at the same instant that a rule would otherwise take effect in
the earlier zone or continuation line, the rule is ignored. In a single zone
it is an error if two rules take effect at the same instant, or if two zone
changes take effect at the same instant.
A link line has the form
Link TARGET LINK-NAME
Link Europe/Istanbul Asia/Istanbul
field should appear as the NAME
field in some zone
line. The LINK-NAME
field is used as an alternative name for that zone;
it has the same syntax as a zone line's NAME
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
Leap 2016 Dec 31 23:59:60 + S
, and HH:MM:SS
fields tell when
the leap second happened. The CORR
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. In this example, the EU rules are for the European Union and for
its predecessor organization, the European Communities.
# 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 UTOFF RULES 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 Europe/Vaduz
In this example, the timezone is named Europe/Zurich but it has an alias as
Europe/Vaduz. This example says that Zurich was 34 minutes and 8 seconds east
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 the UT offset became one hour and Swiss daylight saving
rules (defined with lines beginning with “Rule Swiss”) apply.
From 1981 to the present, EU daylight saving rules have applied, and the UTC
offset has remained at one hour.
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 time zone abbreviation has been CET for standard time and CEST
for daylight saving time.
- Default local timezone file.
- Default timezone information directory.
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
If, for a particular timezone, 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
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