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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 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 incompatibili‐
ties. Although the default is currently fat, this is intended to change in future zic
versions, as software that mishandles the 64-bit data typically mishandles timestamps
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 contained 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This feature is obsolete and poorly supported.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 transitions are at standard time or Universal Time (UT) instead of local
time.

-r [@lo][/@hi]
Reduce the size of output files by limiting their applicability to timestamps in the
range from lo (inclusive) to hi (exclusive), where lo and hi are possibly-signed deci‐
mal counts of seconds since the Epoch (1970-01-01 00:00:00 UTC). Omitted counts de‐
fault 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 intended for past timestamps. Also
see the -b slim option for another 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 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 TZ string. For
example, as of 2019 this problem occurs for Iran's daylight-saving rules for the pre‐
dicted future, as these rules are based on the Iranian calendar, which cannot be rep‐
resented.

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 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 511 bytes, and without any NUL bytes.
The input text's encoding is typically UTF-8 or ASCII; it should have a unibyte representa‐
tion 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 soft‐
ware 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 in‐
clude 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 unambigu‐
ous 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 exten‐
sions, 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 sup‐
plied; 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.

TO 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 field.

TYPE should be “-” and is present for compatibility with older versions 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 neigh‐
boring 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 preci‐
sion. 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 ef‐
fect, 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 omit‐
ted, 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 infor‐
mation 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 day‐
light 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, 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.

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 “-”.

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 ex‐
cept 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 pre‐
vious line in the file used by the previous line. Continuation lines may contain “un‐
til” 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.

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 input. However, the be‐
havior 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 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.

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 expiration 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 offset 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 re‐
mained 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 coin‐
cides 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.