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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 timestamps 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Unless timezone is “-”, this option is obsolete and poorly supported.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., before 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 an extended POSIX.1-2017
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 extended POSIX.1-2017 TZ string. This option does
not affect the represented timestamps. Although it accommodates nonstandard TZif
readers that ignore the extended POSIX.1-2017 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 future cannot be summarized as an extended POSIX.1-2017 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 an extended POSIX.1-2017 TZ string cannot represent.
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.
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 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 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 initial 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 characters from the set
“!$%&'()*,/:;<=>?@[\]^`{|}~”.
FROM 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. 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.
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 previously 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.
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 or spelled out in full. 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. Recognized 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
seconds after local midnight instead of the usual 3600 seconds. 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.
SAVE Gives the amount of time to be added to local standard time when the rule is in
effect, and whether the resulting 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 substring 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 column, giving the amount of time to be added to
local standard time 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.
FORMAT 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 “-”. By convention, the time zone abbreviation “-00” is a
placeholder that means local time is unspecified.
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 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. 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 second 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 second
time given by the other fields should be interpreted as local (wall clock) time.
Rolling leap seconds were implemented back when it was not clear whether common practice
was rolling or stationary, with concerns that one would 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. However, this countdown style does not
seem to have caught on, which means rolling leap seconds are not used in practice; also,
they 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 Communities. 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 earliest 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)