zic(8) man page

ZIC(8) System Manager's Manual ZIC(8)

NAME
zic – timezone compiler

SYNOPSIS
zic [--help] [--version] [-Dsv] [-b slim | fat] [-d directory] [-g gid]
[-l localtime] [-L leapseconds] [-m mode] [-p posixrules] [-r
[@lo][/@hi]] [-R -@hi] [-t localtime-link] [-u uid] [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.

The following options are available:

--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 Do not create directories.

-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. The zic utility 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 filename
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. The zic utility will act
as if the input contained a link line of the form

Link timezone posixrules

Unless timezone is “”, this option is obsolete and poorly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

If timezone is ‘-’, any already-existing link is removed.

-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, -r -@0
omits data intended for negative timestamps (i.e., before the
Epoch), and -r -@0/@2147483648 outputs data intended only for
nonnegative timestamps that fit into 31-bit signed integers.
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 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 TZ string. This option does not affect the represented
timestamps. Although it accommodates nonstandard TZif readers
that ignore the extended POSIX 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 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 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. 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.

- 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°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
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.