Ubuntu Manpage: zic - timezone compiler

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synopsis
description
options
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extended example
files
notes
see also

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Unless timezone is “-”, this option is obsolete and poorly supported.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.

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

FILES

EXTENDED EXAMPLE

FILES

NOTES

SEE ALSO