zic(8) Linux System Administration 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 time conversion information files specified in this in-
put. 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. Although the default is currently
fat, this is intended to change in future zic versions, as soft-
ware that mishandles the 64-bit data typically mishandles time-
stamps after the year 2038 anyway. Also see the -r option for
another way to shrink 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
-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
This feature is obsolete and poorly supported.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 transi-
tions are at standard time or Universal Time (UT) instead of lo-
cal time.
-r [@lo][/@hi]
Reduce the size of output files by limiting their applicability
to timestamps in the range from lo (inclusive) to hi (exclu-
sive), 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. 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 in-
tended for past timestamps. Also see the -b slim option for an-
other way to shrink output size.
-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.
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 TZ string. For example, as of
2019 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 com-
patibility issues affect only timestamps before 1970 or after
the start of 2038.
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 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) ⟨http://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 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 TYPE 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. The word minimum (or an
abbreviation) means the indefinite past. The word maximum (or
an abbreviation) means the indefinite future. Rules can de-
scribe times that are not representable as time values, with
the unrepresentable times ignored; this allows rules to be por-
table among hosts with differing time value types.
TO Gives the final year in which the rule applies. In addition to
minimum and maximum (as above), the word only (or an abbrevia-
tion) may be used to repeat the value of the FROM field.
TYPE should be “-” and is present for compatibility with older ver-
sions of zic in which it could contain year types.
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
format does not specify any maximum precision. Any of these
forms may be followed by the letter w if the given time is lo-
cal 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 indica-
tor, 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 Ire-
land, 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; 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 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.
FORMAT
The format for time zone abbreviations. The pair of characters
%s is used to show where the “variable part” of the time zone ab-
breviation 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 (/) separates
standard and daylight abbreviations. To conform to POSIX, a time
zone abbreviation should contain only alphanumeric ASCII charac-
ters, “+” and “-”.
UNTIL The time at which the UT offset or the rule(s) change for a loca-
tion. 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. 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.
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.
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.
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, or if the source of one link line is the
target of another.
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.
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; zic outputs this expiration timestamp
by truncating the end of the output file to the timestamp. If there is
no expiration line, zic also accepts a comment “#expires E ...” where E
is the expiration timestamp as a decimal integer count of seconds since
the Epoch, not counting leap seconds. However, the “#expires” comment
is an obsolescent feature, and the leap second file should use an expi-
ration line instead of relying on a comment.
EXTENDED EXAMPLE
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 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 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 off-
set was changed to 7°26′22.50″, 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 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)
2020-08-13 zic(8)
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