Manual: zic
zic(8) System Manager's Manual zic(8)
NAME
zic - timezone compiler
SYNOPSIS
zic [ option ... ] [ filename ... ]
DESCRIPTION
The zic program reads text from the file(s) named on the command line and creates the timezone information format
(TZif) files specified in this input. If a filename is “-”, standard input is read.
OPTIONS
--version
Output version information and exit.
--help Output short usage message and exit.
-b bloat
Output backward-compatibility data as specified by bloat. If bloat is fat, generate additional data entries
that work around potential bugs or incompatibilities in older software, such as software that mishandles the
64-bit generated data. If bloat is slim, keep the output files small; this can help check for the bugs and
incompatibilities. The default is slim, as software that mishandles 64-bit data typically mishandles time‐
stamps after the year 2038 anyway. Also see the -r option for another way to alter output size.
-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
If timezone is -, any already-existing link is removed.
-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 files.
-p timezone
Use timezone's rules when handling nonstandard TZ strings like "EET-2EEST" that lack transition rules. zic
will act as if the input contained a link line of the form
Link timezone posixrules
If timezone is “-” (the default), any already-existing link is removed.
Unless timezone is “-”, this option is obsolete and poorly supported. Among other things it should not be
used for timestamps after the year 2037, and it should not be combined with -b slim if timezone's transitions
are at standard time or Universal Time (UT) instead of local time.
-r [@lo][/@hi]
Limit the applicability of output files to timestamps in the range from lo (inclusive) to hi (exclusive),
where lo and hi are possibly signed decimal counts of seconds since the Epoch (1970-01-01 00:00:00 UTC).
Omitted counts default to extreme values. The output files use UT offset 0 and abbreviation “-00” in place of
the omitted timestamp data. For example, “zic -r @0” omits data intended for negative timestamps (i.e., be‐
fore the Epoch), and “zic -r @0/@2147483648” outputs data intended only for nonnegative timestamps that fit
into 31-bit signed integers. On platforms with GNU date, “zic -r @$(date +%s)” omits data intended for past
timestamps. Although this option typically reduces the output file's size, the size can increase due to the
need to represent the timestamp range boundaries, particularly if hi causes a TZif file to contain explicit
entries for pre-hi transitions rather than concisely representing them with a proleptic TZ string. Also see
the -b slim option for another way to shrink output size.
-R @hi Generate redundant trailing explicit transitions for timestamps that occur less than hi seconds since the
Epoch, even though the transitions could be more concisely represented via the proleptic TZ string. This op‐
tion does not affect the represented timestamps. Although it accommodates nonstandard TZif readers that ig‐
nore the proleptic TZ string, it increases the size of the altered output files.
-t file
When creating local time information, put the configuration link in the named file rather than in the standard
location.
-v Be more verbose, and complain about the following situations:
The input specifies a link to a link, something not supported by some older parsers, including zic itself
through release 2022e.
A year that appears in a data file is outside the range of representable years.
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.
A time zone abbreviation uses a %z format. Pre-2015 versions of zic do not support this.
A timestamp contains fractional seconds. Pre-2018 versions of zic do not support this.
The input contains abbreviations that are mishandled by pre-2018 versions of zic due to a longstanding coding
bug. These abbreviations include “L” for “Link”, “mi” for “min”, “Sa” for “Sat”, and “Su” for “Sun”.
The output file does not contain all the information about the long-term future of a timezone, because the fu‐
ture cannot be summarized as a proleptic TZ string. For example, as of 2023 this problem occurs for Morocco's
daylight-saving rules, as these rules are based on predictions for when Ramadan will be observed, something
that a proleptic TZ string cannot represent.
The output contains data that may not be handled properly by client code designed for older zic output for‐
mats. These compatibility issues affect only timestamps before 1970 or after the start of 2038.
The output contains a truncated leap second table, which can cause some older TZif readers to misbehave. This
can occur if the -L option is used, and either an Expires line is present or the -r option is also used.
The output file contains more than 1200 transitions, which may be mishandled by some clients. The current
reference client supports at most 2000 transitions; pre-2014 versions of the reference client support at most
1200 transitions.
A time zone abbreviation has fewer than 3 or more than 6 characters. POSIX requires at least 3, and requires
implementations to support at least 6.
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 “-”.
FILES
Input files use the format described in this section; output files use tzfile(5) format.
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 2048 bytes counting the newline, 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)
⟨https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap06.html⟩ 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 option.
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 ex‐
tends to the end of the line the sharp character appears on. White space characters and sharp characters may be en‐
closed in double quotes (") if they're to be used as part of a field. Any line that is blank (after comment strip‐
ping) is ignored. Nonblank lines are expected to be of one of three types: rule lines, zone lines, and link lines.
Names must be in English and are case insensitive. They appear in several contexts, and include month and weekday
names and keywords such as maximum, only, Rolling, and Zone. A name can be abbreviated by omitting all but an ini‐
tial prefix; any abbreviation must be unambiguous in context.
A rule line has the form
Rule NAME FROM TO - IN ON AT SAVE LETTER/S
For example:
Rule US 1967 1973 - Apr lastSun 2:00w 1:00d D
The fields that make up a rule line are:
NAME 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 charac‐
ters from the set “!$%&'()*,/:;<=>?@[\]^`{|}~”.
FROM Gives the first year in which the rule applies. Any signed integer year can be supplied; the proleptic Grego‐
rian calendar is assumed, with year 0 preceding year 1. 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 dif‐
fering time value types.
TO Gives the final year in which the rule applies. The word maximum (or an abbreviation) means the indefinite
future, and the word only (or an abbreviation) may be used to repeat the value of the FROM field.
- Is a reserved field and should always contain “-” for compatibility with older versions of zic. It was previ‐
ously known as the TYPE field, which could contain values to allow a separate script to further restrict in
which “types” of years the rule would apply.
IN Names the month in which the rule takes effect. Month names may be abbreviated as mentioned previously; for
example, January can appear as “January”, “JANU” or “Ja”, but not as “j” which would be ambiguous with both
June and July.
ON 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 as
mentioned previously, e.g., “Su” for Sunday and “lastsa” for the last Saturday. There must be no white space
characters within the ON field. The “<=” and “>=” constructs can result in a day in the neighboring month;
for example, the IN-ON combination “Oct Sun>=31” stands for the first Sunday on or after October 31, even if
that Sunday occurs in November.
AT Gives the time of day at which the rule takes effect, relative to 00:00, the start of a calendar day. Recog‐
nized forms include:
2 time in hours
2:00 time in hours and minutes
01:28:14 time in hours, minutes, and seconds
00:19:32.13 time with fractional seconds
12:00 midday, 12 hours after 00:00
15:00 3 PM, 15 hours after 00:00
24:00 end of day, 24 hours after 00:00
260:00 260 hours after 00:00
-2:30 2.5 hours before 00:00
- equivalent to 0
Although zic rounds times to the nearest integer second (breaking ties to the even integer), the fractions may
be useful to other applications requiring greater precision. The source format does not specify any maximum
precision. Any of these forms may be followed by the letter w if the given time is local or “wall clock”
time, s if the given time is standard time without any adjustment for daylight saving, or u (or g or z) if the
given time is universal time; in the absence of an indicator, local (wall clock) time is assumed. These forms
ignore leap seconds; for example, if a leap second occurs at 00:59:60 local time, “1:00” stands for 3601 sec‐
onds after local midnight instead of the usual 3600 seconds. The intent is that a rule line describes the in‐
stants when a clock/calendar set to the type of time specified in the AT field would show the specified date
and time of day.
SAVE Gives the amount of time to be added to local standard time when the rule is in effect, and whether the re‐
sulting time is standard or daylight saving. This field has the same format as the AT field except with a
different set of suffix letters: s for standard time and d for daylight saving time. The suffix letter is
typically omitted, and defaults to s if the offset is zero and to d otherwise. 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.
LETTER/S
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 STDOFF RULES FORMAT [UNTIL]
For example:
Zone Asia/Amman 2:00 Jordan EE%sT 2017 Oct 27 01:00
The fields that make up a zone line are:
NAME 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 sub‐
string that does not contain “/”.
STDOFF The amount of time to add to UT to get standard time, without any adjustment for daylight saving. This field
has the same format as the AT and SAVE fields of rule lines, except without suffix letters; begin the field
with a minus sign if time must be subtracted from UT.
RULES The name of the rules that apply in the timezone or, alternatively, a field in the same format as a rule-line
SAVE field, giving the amount of time to be added to local standard time and whether the resulting time is
standard or daylight saving. Standard time applies if this field is - or for timestamps occurring before any
rule takes effect. When an amount of time is given, only the sum of standard time and this amount matters.
FORMAT The format for time zone abbreviations. The pair of characters %s shows where to put the time zone abbrevia‐
tion's variable part, which is taken from the LETTER/S field of the corresponding rule; any timestamps that
precede the earliest rule use the LETTER/S of the earliest standard-time rule (which in this case must exist).
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, min‐
utes, and seconds east (+) or west (-) of UT. Alternatively, a slash (/) separates standard and daylight ab‐
breviations. To conform to POSIX, a time zone abbreviation should contain only alphanumeric ASCII characters,
“+” and “-”. By convention, the time zone abbreviation “-00” is a placeholder that means local time is un‐
specified.
UNTIL The time at which the UT offset or the rule(s) change for a location. It takes the form of one to four fields
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 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 speci‐
fied 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 con‐
tinuation.
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. A zone or continuation line L with a named rule set starts with standard time by default:
that is, any of L's timestamps preceding L's earliest rule use the rule in effect after L's first transition into
standard time. 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.
If a continuation line subtracts N seconds from the UT offset after a transition that would be interpreted to be
later if using the continuation line's UT offset and rules, the “until” time of the previous zone or continuation
line is interpreted according to the continuation line's UT offset and rules, and any rule that would otherwise take
effect in the next N seconds is instead assumed to take effect simultaneously. For example:
# Rule NAME FROM TO - IN ON AT SAVE LETTER/S
Rule US 1967 2006 - Oct lastSun 2:00 0 S
Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
# Zone NAME STDOFF RULES FORMAT [UNTIL]
Zone America/Menominee -5:00 - EST 1973 Apr 29 2:00
-6:00 US C%sT
Here, an incorrect reading would be there were two clock changes on 1973-04-29, the first from 02:00 EST (-05) to
01:00 CST (-06), and the second an hour later from 02:00 CST (-06) to 03:00 CDT (-05). However, zic interprets this
more sensibly as a single transition from 02:00 CST (-05) to 02:00 CDT (-05).
A link line has the form
Link TARGET LINK-NAME
For example:
Link Europe/Istanbul Asia/Istanbul
The TARGET field should appear as the NAME field in some zone line or as the LINK-NAME field in some link 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 field.
Links can chain together, although the behavior is unspecified if a chain of one or more links does not terminate in
a Zone name. A link line can appear before the line that defines the link target. For example:
Link Greenwich G_M_T
Link Etc/GMT Greenwich
Zone Etc/GMT 0 - GMT
The two links are chained together, and G_M_T, Greenwich, and Etc/GMT all name the same zone.
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.
The file that describes leap seconds can have leap lines and an expiration line. Leap lines have the following form:
Leap YEAR MONTH DAY HH:MM:SS CORR R/S
For example:
Leap 2016 Dec 31 23:59:60 + S
The YEAR, MONTH, DAY, and HH:MM:SS fields tell when the leap second happened. The CORR field should be “+” if a sec‐
ond was added or “-” if a second was skipped. The R/S 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 sec‐
ond time given by the other fields should be interpreted as local (wall clock) time.
Rolling leap seconds would let one see Times Square ball drops where there'd be a “3... 2... 1... leap... Happy New
Year” countdown, placing the leap second at midnight New York time rather than midnight UTC. Although stationary
leap seconds are the common practice, rolling leap seconds can be useful in specialized applications like SMPTE time‐
codes that may prefer to put leap second discontinuities at the end of a local broadcast day. However, rolling leap
seconds are not supported if the -r option is used.
The expiration line, if present, has the form:
Expires YEAR MONTH DAY HH:MM:SS
For example:
Expires 2020 Dec 28 00:00:00
The YEAR, MONTH, DAY, and HH:MM:SS fields give the expiration timestamp in UTC for the leap second table.
EXTENDED EXAMPLE
Here is an extended example of zic input, intended to illustrate many of its features.
# Rule NAME FROM TO - 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 STDOFF RULES FORMAT [UNTIL]
Zone Europe/Zurich 0:34:08 - LMT 1853 Jul 16
0:29:45.50 - BMT 1894 Jun
1:00 Swiss CE%sT 1981
1:00 EU CE%sT
Link Europe/Zurich Europe/Vaduz
In this example, the EU rules are for the European Union and for its predecessor organization, the European Communi‐
ties. The timezone is named Europe/Zurich and it has the alias 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 degrees 26 minutes
22.50 seconds, which works out to 0:29:45.50; zic treats this by rounding it to 0:29:46. 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.
FILES
/etc/localtime
Default local timezone file.
/usr/share/zoneinfo
Default timezone information directory.
NOTES
For areas with more than two types of local time, you may need to use local standard time in the AT field of the ear‐
liest transition time's rule to ensure that the earliest transition time recorded in the compiled file is correct.
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 offset, zic produces a single transition to daylight saving at the new UT
offset without any change in local (wall clock) time. To get separate transitions use multiple zone continuation
lines specifying transition instants using universal time.
SEE ALSO
tzfile(5), zdump(8)
Time Zone Database zic(8)