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 timestamps af-
ter 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 con-
tained 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Unless timezone is “-”, this option is obsolete and poorly sup-
ported. 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 in-
tegers. On platforms with GNU date, “zic -r @$(date +%s)” omits
data intended for past timestamps. Although this option typi-
cally reduces the output file's size, the size can increase due
to the need to represent the timestamp range boundaries, partic-
ularly 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 repre-
sented timestamps. Although it accommodates nonstandard TZif
readers that ignore the extended POSIX.1-2017 TZ string, it in-
creases 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 rep-
resentable 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 ver-
sions 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 abbre-
viations 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 com-
patibility 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 ref-
erence client support at most 1200 transitions.
A time zone abbreviation has fewer than 3 or more than 6 charac-
ters. 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 con-
tains 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 en-
tirely 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 an-
other by one or more white space characters. The white space charac-
ters 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 sev-
eral contexts, and include month and weekday names and keywords such as
maximum, only, Rolling, and Zone. A name can be abbreviated by omit-
ting 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 in-
teger 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 unrepre-
sentable 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 maxi-
mum (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 compati-
bility 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 in-
clude:
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 (break-
ing ties to the even integer), the fractions may be useful to
other applications requiring greater precision. The source for-
mat 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 in-
stead 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 stan-
dard 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 dis-
tinguish 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 com-
ponent 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, alterna-
tively, 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 informa-
tion, where hh, mm, and ss are the hours, minutes, and seconds
east (+) or west (-) of UT. Alternatively, a slash (/) sepa-
rates standard and daylight abbreviations. To conform to POSIX,
a time zone abbreviation should contain only alphanumeric ASCII
characters, “+” and “-”. By convention, the time zone abbrevia-
tion “-00” is a placeholder that means local time is unspeci-
fied.
UNTIL The time at which the UT offset or the rule(s) change for a lo-
cation. 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 pos-
sible 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. Continua-
tion 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 stan-
dard time by default: that is, any of L's timestamps preceding L's ear-
liest rule use the rule in effect after L's first transition into stan-
dard 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 in-
stant.
If a continuation line subtracts N seconds from the UT offset after a
transition that would be interpreted to be later if using the continua-
tion 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 ef-
fect in the next N seconds is instead assumed to take effect simultane-
ously. 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 in-
put. 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 expira-
tion 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 mid-
night 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 be-
came one hour and Swiss daylight saving rules (defined with lines be-
ginning with “Rule Swiss”) apply. From 1981 to the present, EU day-
light 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 com-
pleteness. 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, respec-
tively. 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 com-
piled 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)
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