xref: /netbsd-src/lib/libc/time/zic.8 (revision 1b58480149d5fc503578a6ef5ed0abc09050cbed)
1.\"	$NetBSD: zic.8,v 1.37 2021/10/22 14:26:04 christos Exp $
2.Dd Fabruary 28, 2021
3.Dt ZIC 8
4.Os
5.Sh NAME
6.Nm zic
7.Nd timezone compiler
8.Sh SYNOPSIS
9.Nm
10.Op Fl \-version
11.Op Fl \-help
12.Op Fl b
13.Op Fl d Ar directory
14.Op Fl L Ar leapsecondfilename
15.Op Fl l Ar localtime
16.Op Fl p Ar posixrules
17.Op Fl s
18.Op Fl t Ar file
19.Op Fl v
20.Op Fl y Ar command
21.Op Ar Filename ...
22.Sh DESCRIPTION
23The
24.Nm
25program reads text from the file(s) named on the command line
26and creates the timezone information format (TZif) files
27specified in this input.
28If a
29.Ar filename
30is
31.Ar \&- ,
32standard input is read.
33.Pp
34.Sh OPTIONS
35.Bl -tag -width XXXXXXXXXX -compact
36.It Fl \-version
37Output version information and exit.
38.It Fl \-help
39Output short usage message and exit.
40.It Fl b Ar bloat
41Output backward-compatibility data as specified by
42.Ar bloat .
43If
44.Ar bloat
45is
46.Dv fat ,
47generate additional data entries that work around potential bugs or
48incompatibilities in older software, such as software that mishandles
49the 64-bit generated data.
50If
51.Ar bloat
52is
53.Dv slim ,
54keep the output files small; this can help check for the bugs
55and incompatibilities.
56The default is
57.Dv slim ,
58as software that mishandles 64-bit data typically
59Also see the
60.Fl r
61option for another way to alter output size.
62.It Fl d Ar directory
63Create time conversion information files in the named directory rather than
64in the standard directory named below.
65.It Fl l Ar timezone
66Use the
67.Ar timezone
68as local time.
69.Nm
70will act as if the input contained a link line of the form
71.Dl Link	timezone	localtime
72If
73.Ar timezone
74is
75.Dv \&- ,
76any already-existing link is removed.
77.It Fl L Ar leapsecondfilename
78Read leap second information from the file with the given name.
79If this option is not used,
80no leap second information appears in output files.
81.It Fl p Ar timezone
82Use
83.Ar timezone's
84rules when handling POSIX-format
85TZ strings like
86.Qq EET\&-2EEST
87that lack transition rules.
88.Nm
89will act as if the input contained a link line of the form
90.Dl Link	timezone	posixrules
91.Pp
92This feature is obsolete and poorly supported.
93Among other things it should not be used for timestamps after the year 2037,
94and it should not be combined with
95.Fl b Ar slim
96if
97.Va timezone's
98transitions are at standard time or Universal Time (UT) instead of local time.
99If
100.Ar timezone
101is
102.Dv \&- ,
103any already-existing link is removed.
104.It Fl r Op Ar @lo / Op Ar @hi
105Limit the applicability of output files
106to timestamps in the range from
107.Ar lo
108(inclusive) to
109.Ar hi
110(exclusive), where
111.Ar lo
112and
113.Ar hi
114are possibly-signed decimal counts of seconds since the Epoch
115(1970-01-01 00:00:00 UTC).
116Omitted counts default to extreme values.
117The output files use UT offset 0 and abbreviation
118.q "\*-00"
119in place of the omitted timestamp data;
120this typically reduces the files' sizes.
121For example,
122.Bd literal
123zic -r @0
124.Ed
125omits data intended for negative timestamps (i.e., before the Epoch), and
126.Bd literal
127zic -r @0/@2147483648
128.Ed
129outputs data intended only for nonnegative timestamps that fit into
13031-bit signed integers.
131Or using
132.Xr date 1 ,
133.Bd literal
134zic -r @$(date +%s)
135.Ed
136omits data intended for past timestamps.
137Also see the
138.Fl b Ar slim
139option for another way to shrink output size.
140.It Fl t Ar file
141When creating local time information, put the configuration link in
142the named file rather than in the standard location.
143.It Fl v
144Be more verbose, and complain about the following situations:
145.Bl -dash
146.It
147The input specifies a link to a link.
148.It
149A year that appears in a data file is outside the range
150of representable years.
151.It
152A time of 24:00 or more appears in the input.
153Pre-1998 versions of
154.Nm
155prohibit 24:00, and pre-2007 versions prohibit times greater than 24:00.
156.It
157A rule goes past the start or end of the month.
158Pre-2004 versions of
159.Nm
160prohibit this.
161.It
162A time zone abbreviation uses a
163.Dv %z
164format.
165Pre-2015 versions of
166.Nm
167do not support this.
168.It
169A timestamp contains fractional seconds.
170Pre-2018 versions of
171.Nm
172do not support this.
173.It
174The input contains abbreviations that are mishandled by pre-2018 versions of
175.Nm
176due to a longstanding coding bug.
177These abbreviations include
178.Qq L
179for
180.Qq Link ,
181.Qq mi
182for
183.Qq min ,
184.Qq Sa
185for
186.Qq Sat ,
187and
188.Qq Su
189for
190.Qq Sun .
191.It
192The output file does not contain all the information about the
193long-term future of a timezone, because the future cannot be summarized as
194an extended POSIX TZ string.
195For example, as of 2019 this problem
196occurs for Iran's daylight-saving rules for the predicted future, as
197these rules are based on the Iranian calendar, which cannot be
198represented.
199.It
200The output contains data that may not be handled properly by client
201code designed for older
202.Xr zic 8
203output formats.
204These compatibility issues affect only timestamps
205before 1970 or after the start of 2038.
206.It
207The output contains a truncated leap second table,
208which can cause some older TZif readers to misbehave.
209This can occur if the
210.Fl L
211option is used, and either an Expires line is present or
212the
213.Fl r
214option is also used.
215.It
216The output file contains more than 1200 transitions,
217which may be mishandled by some clients.
218The current reference client supports at most 2000 transitions;
219pre-2014 versions of the reference client support at most 1200
220transitions.
221.It
222A time zone abbreviation has fewer than 3 or more than 6 characters.
223POSIX requires at least 3, and requires implementations to support
224at least 6.
225.It
226An output file name contains a byte that is not an ASCII letter,
227.Qq - ,
228.Qq / ,
229or
230.Qq _ ;
231or it
232or it contains a file name component that contains more than 14 bytes
233or that starts with
234.Qq - .
235.El
236.El
237.Pp
238Input files should be text files, that is, they should be a series of
239zero or more lines, each ending in a newline byte and containing at
240most 511 bytes, and without any
241.Dv NUL
242bytes.
243The input text's encoding
244is typically UTF-8 or ASCII; it should have a unibyte representation
245for the POSIX Portable Character Set (PPCS)
246.Rs
247.%U https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap06.html
248.Re
249and the encoding's non-unibyte characters should consist entirely of
250non-PPCS bytes.
251Non-PPCS characters typically occur only in comments:
252although output file names and time zone abbreviations can contain
253nearly any character, other software will work better if these are
254limited to the restricted syntax described under the
255.Op v
256option.
257.Pp
258Input lines are made up of fields.
259Fields are separated from one another by one or more white space characters.
260The white space characters are space, form feed, carriage return, newline,
261tab, and vertical tab.
262Leading and trailing white space on input lines is ignored.
263An unquoted sharp character (#) in the input introduces a comment which extends
264to the end of the line the sharp character appears on.
265White space characters and sharp characters may be enclosed in double
266quotes
267.Pq \&"
268.\" XXX "
269if they're to be used as part of a field.
270Any line that is blank (after comment stripping) is ignored.
271Nonblank lines are expected to be of one of three types:
272rule lines, zone lines, and link lines.
273.Pp
274Names must be in English and are case insensitive.
275They appear in several contexts, and include month and weekday names
276and keywords such as
277.Qq maximum ,
278.Qq only ,
279.Qq Rolling ,
280and
281.Qq Zone .
282A name can be abbreviated by omitting all but an initial prefix; any
283abbreviation must be unambiguous in context.
284.Pp
285A rule line has the form
286.Pp
287.Dl Rule	NAME	FROM	TO	\&-	IN	ON		AT		SAVE		LETTER/S
288.Pp
289For example:
290.Pp
291.Dl Rule	US	1967	1973	\&-	Apr	lastSun	2:00w	1:00d	D
292.Pp
293The fields that make up a rule line are:
294.Bl -tag -width "LETTER/S"
295.It NAME
296Gives the name of the rule set that contains this line.
297The name must start with a character that is neither
298an ASCII digit nor
299.Ar \&-
300nor
301.Ar + .
302To allow for future extensions,
303an unquoted name should not contain characters from the set
304.Ar !$%&'()*,/:;<=>?@[\e]^`{|}~ .
305.It FROM
306Gives the first year in which the rule applies.
307Any signed integer year can be supplied; the proleptic Gregorian calendar
308is assumed, with year 0 preceding year 1.
309The word
310.Em minimum
311(or an abbreviation) means the indefinite past.
312The word
313.Em maximum
314(or an abbreviation) means the indefinite future.
315Rules can describe times that are not representable as time values,
316with the unrepresentable times ignored; this allows rules to be portable
317among hosts with differing time value types.
318.It TO
319Gives the final year in which the rule applies.
320In addition to
321.Em minimum
322and
323.Em maximum
324(as above),
325the word
326.Em only
327(or an abbreviation)
328may be used to repeat the value of the
329.Em FROM
330field.
331.It \&-
332should be
333.Qq \&-
334for compatibility with older versions of
335.Nm .
336It was previously known as the
337.Em TYPE
338field, which could contain values to allow a
339separate script to further restrict in which
340.Em types
341of years the rule would apply.
342.It IN
343Names the month in which the rule takes effect.
344Month names may be abbreviated.
345.It ON
346Gives the day on which the rule takes effect.
347Recognized forms include:
348.Pp
349.Bl -tag -width lastSun -compact -offset indent
350.It 5
351the fifth of the month
352.It lastSun
353the last Sunday in the month
354.It lastMon
355the last Monday in the month
356.It Sun\*[Ge]8
357first Sunday on or after the eighth
358.It Sun\*[Le]25
359last Sunday on or before the 25th
360.El
361.Pp
362Names of days of the week may be abbreviated or spelled out in full.
363A weekday name (e.g.,
364.Qq Sunday )
365or a weekday name preceded by
366.Qq last
367(e.g.,
368.Qq lastSunday )
369may be abbreviated or spelled out in full.
370There must be no white space characters within the
371.Em ON
372field.
373The
374.Qq <=
375and
376.Qq >=
377constructs can result in a day in the neighboring month;
378for example, the IN-ON combination
379.Qq "Oct Sun>=31"
380tands for the first Sunday on or after October 31,
381even if that Sunday occurs in November.
382.It AT
383Gives the time of day at which the rule takes effect,
384relative to 00:00, the start of a calendar day.
385Recognized forms include:
386.Pp
387.Bl -tag -width "00X19X32X13" -compact -offset indent
388.It 2
389time in hours
390.It 2:00
391time in hours and minutes
392.It 01:28:14
393time in hours, minutes, and seconds
394.It 00:19:32.13
395time with fractional seconds
396.It 12:00
397midday, 12 hours after 00:00
398.It 15:00
3993 PM, 15 hours after 00:00
400.It 24:00
401end of day, 24 hours after 00:00
402.It 260:00
403260 hours after 00:00
404.It \-2:30
4052.5 hours before 00:00
406.It \-
407equivalent to 0
408.El
409.Pp
410Although
411.I zic
412rounds times to the nearest integer second
413(breaking ties to the even integer), the fractions may be useful
414to other applications requiring greater precision.
415The source format does not specify any maximum precision.
416Any of these forms may be followed by the letter
417.Em w
418if the given time is local or
419.Qq wall clock
420time,
421.Em s
422if the given time is standard time without any adjustment for daylight saving,
423or
424.Em u
425(or
426.Em g
427or
428.Em z )
429if the given time is universal time;
430in the absence of an indicator,
431local (wall clock) time is assumed.
432These forms ignore leap seconds; for example,
433if a leap second occurs at 00:59:60 local time,
434.q "1:00"
435stands for 3601 seconds after local midnight instead of the usual 3600 seconds.
436The intent is that a rule line describes the instants when a
437clock/calendar set to the type of time specified in the
438.Em AT
439field would show the specified date and time of day.
440.It SAVE
441Gives the amount of time to be added to local standard time when the rule is in
442effect, and whether the resulting time is standard or daylight saving.
443This field has the same format as the
444.Em AT
445field
446.Em s
447for standard time and
448.Em d
449for daylight saving time.
450The suffix letter is typically omitted, and defaults to
451.Em s
452if the offset is zero and to
453.Em d
454otherwise.
455Negative offsets are allowed; in Ireland, for example, daylight saving
456time is observed in winter and has a negative offset relative to
457Irish Standard Time.
458The offset is merely added to standard time; for example,
459.Nm
460does not distinguish a 10:30 standard time plus an 0:30
461.Em SAVE
462from a 10:00 standard time plus a 1:00
463.Em SAVE .
464.It LETTER/S
465Gives the
466.Qq variable part
467(for example, the
468.Qq S
469or
470.Qq D
471in
472.Qq EST
473or
474.Qq EDT )
475of time zone abbreviations to be used when this rule is in effect.
476If this field is
477.Em \&- ,
478the variable part is null.
479.El
480.Pp
481A zone line has the form
482.Pp
483.Dl Zone	NAME			STDOFF	RULES/SAVE	FORMAT	[UNTIL]
484.Pp
485For example:
486.Pp
487.Dl Zone	Asia/Amman	2:00	Jordan	EE%sT	2017 Oct 27 1:00
488.Pp
489The fields that make up a zone line are:
490.Bl -tag -width "RULES/SAVE"
491.It NAME
492The name of the timezone.
493This is the name used in creating the time conversion information file for the
494timezone.
495It should not contain a file name component
496.Qq .
497or
498.Qq .. ;
499a file name component is a maximal substring that does not contain
500.Qq / .
501.It STDOFF
502The amount of time to add to UT to get standard time,
503without any adjustment for daylight saving.
504This field has the same format as the
505.Em AT
506and
507.Em SAVE
508fields of rule lines;
509begin the field with a minus sign if time must be subtracted from UT.
510.It RULES
511The name of the rules that apply in the timezone or,
512alternatively, a field in the same format as a rule-line SAVE column,
513giving the amount of time to be added to local standard time
514and whether the resulting time is standard or daylight saving.
515If this field is
516.Em \&-
517then standard time always applies.
518When an amount of time is given, only the sum of standard time and
519this amount matters.
520.It FORMAT
521The format for time zone abbreviations.
522The pair of characters
523.Em %s
524is used to show where the
525.Qq variable part
526of the time zone abbreviation goes.
527Alternatively, a format can use the pair of characters
528.Em %z
529+to stand for the UT offset in the form
530.Em \(+- hh ,
531.Em \(+- hhmm ,
532or
533.Em \(+- hhmmss ,
534using the shortest form that does not lose information, where
535.Em hh ,
536.Em mm ,
537and
538.Em ss
539are the hours, minutes, and seconds east (+) or west (\-) of UT.
540Alternatively,
541a slash
542.Pq \&/
543separates standard and daylight abbreviations.
544To conform to POSIX, a time zone abbreviation should contain only
545alphanumeric ASCII characters,
546.Qq +
547and
548.Qq \&- .
549By convention, the time zone abbreviation
550.Qq \&-00
551is a placeholder that means local time is unspecified.
552.It UNTIL
553The time at which the UT offset or the rule(s) change for a location.
554It takes the form of one to four fields YEAR [MONTH [DAY [TIME]]].
555If this is specified,
556the time zone information is generated from the given UT offset
557and rule change until the time specified, which is interpreted using
558the rules in effect just before the transition.
559The month, day, and time of day have the same format as the IN, ON, and AT
560fields of a rule; trailing fields can be omitted, and default to the
561earliest possible value for the missing fields.
562.Pp
563The next line must be a
564.Qq continuation
565line; this has the same form as a zone line except that the
566string
567.Qq Zone
568and the name are omitted, as the continuation line will
569place information starting at the time specified as the
570.Em until
571information in the previous line in the file used by the previous line.
572Continuation lines may contain
573.Em until
574information, just as zone lines do, indicating that the next line is a further
575continuation.
576.El
577.Pp
578If a zone changes at the same instant that a rule would otherwise take
579effect in the earlier zone or continuation line, the rule is ignored.
580A zone or continuation line
581.I L
582with a named rule set starts with standard time by default:
583that is, any of
584.IR L 's
585timestamps preceding
586.IR L 's
587earliest rule use the rule in effect after
588.IR L 's
589first transition into standard time.
590In a single zone it is an error if two rules take effect at the same
591instant, or if two zone changes take effect at the same instant.
592.Pp
593If a continuation line subtracts
594.Dv N
595seconds from the UT offset after a transition that would be
596interpreted to be later if using the continuation line's UT offset and
597rules, the
598.Em until
599time of the previous zone or continuation line is interpreted
600according to the continuation line's UT offset and rules, and any rule
601that would otherwise take effect in the next
602.Dv N
603seconds is instead assumed to take effect simultaneously.
604For example:
605.Pp
606.Bl -column -compact -offset indent "# Rule" "Swiss" "FROM" "1995" "\&*" "Oct" "lastSun" "1:00u" "SAVE" "LETTER/S"
607.It # Rule	NAME	FROM	TO	\&-	IN	ON	AT	SAVE	LETTER/S
608.It Rule	US	1967	2006	\&-	Oct	lastSun	2:00	0	S
609.It Rule	US	1967	1973	\&-	Apr	lastSun	2:00	1:00	D
610.It # Zone	NAME				STDOFF	RULES	FORMAT	[UNTIL]
611.It Zone	America/Menominee	\&-5:00	\&-	EST	1973 Apr 29 2:00
612.It						\&-6:00	US	C%sT
613.El
614.Pp
615Here, an incorrect reading would be there were two clock changes on 1973-04-29,
616the first from 02:00 EST (\&-05) to 01:00 CST (\&-06),
617and the second an hour later from 02:00 CST (\&-06) to 03:00 CDT (\&-05).
618However,
619.Nm
620interprets this more sensibly as a single transition from 02:00 CST (\&-05) to
62102:00 CDT (\&-05).
622.Pp
623A link line has the form
624.Pp
625.Dl Link	TARGET			LINK-NAME
626.Pp
627For example:
628.Pp
629.Dl Link	Europe/Istanbul	Asia/Istanbul
630.Pp
631The
632.Em TARGET
633field should appear as the
634.Em NAME
635field in some zone line.
636The
637.Em LINK-NAME
638field is used as an alternative name for that zone;
639it has the same syntax as a zone line's
640.Em NAME
641field.
642.Pp
643Except for continuation lines,
644lines may appear in any order in the input.
645However, the behavior is unspecified if multiple zone or link lines
646define the same name, or if the source of one link line is the target
647of another.
648.Pp
649The file that describes leap seconds can have leap lines and an
650expiration line.
651Leap lines have the following form:
652.Pp
653.Dl Leap	YEAR	MONTH	DAY	HH:MM:SS	CORR	R/S
654.Pp
655For example:
656.Pp
657.Dl Leap	2016	Dec		31	23:59:60	+	S
658.Pp
659The
660.Em YEAR ,
661.Em MONTH ,
662.Em DAY ,
663and
664.Em HH:MM:SS
665fields tell when the leap second happened.
666The
667.Em CORR
668field
669should be
670.Qq \&+
671if a second was added
672or
673.Qq \&-
674if a second was skipped.
675The
676.Em R/S
677field
678should be (an abbreviation of)
679.Qq Stationary
680if the leap second time given by the other fields should be interpreted as UTC
681or
682(an abbreviation of)
683.Qq Rolling
684if the leap second time given by the other fields should be interpreted as
685local (wall clock) time.
686.Pp
687Rolling leap seconds were implemented back when it was not
688clear whether common practice was rolling or stationary,
689with concerns that one would see
690Times Square ball drops where there'd be a
691.Qq 3... 2... 1... leap... Happy New Year
692countdown, placing the leap second at
693midnight New York time rather than midnight UTC.
694However, this countdown style does not seem to have caught on,
695which means rolling leap seconds are not used in practice;
696also, they are not supported if the
697.Fl r
698option is used.
699.Pp
700The expiration line, if present, has the form:
701.Pp
702.Dl Expires	YEAR	MONTH	DAY	HH:MM:SS
703.Pp
704For example:
705.Pp
706.Dl Expires	2020	Dec		28	00:00:00
707.Pp
708The
709.Em YEAR ,
710.Em MONTH ,
711.Em DAY ,
712and
713.Em HH:MM:SS
714fields give the expiration timestamp in UTC for the leap second table;
715.Sh EXTENDED EXAMPLE
716Here is an extended example of
717.Ic zic
718input, intended to illustrate many of its features.
719In this example, the EU rules are for the European Union
720and for its predecessor organization, the European Communities.
721.Pp
722.Bl -column -compact -offset indent "# Rule" "Swiss" "FROM" "1995" "\&*" "Oct" "lastSun" "1:00u" "SAVE" "LETTER/S"
723.It # Rule	NAME	FROM	TO	\&-	IN	ON	AT	SAVE	LETTER/S
724.It Rule	Swiss	1941	1942	\&-	May	Mon>=1	1:00	1:00	S
725.It Rule	Swiss	1941	1942	\&-	Oct	Mon>=1	2:00	0	-
726.Pp
727.It Rule	EU	1977	1980	\&-	Apr	Sun>=1	1:00u	1:00	S
728.It Rule	EU	1977	only	\&-	Sep	lastSun	1:00u	0	-
729.It Rule	EU	1978	only	\&-	Oct	 1	1:00u	0	-
730.It Rule	EU	1979	1995	\&-	Sep	lastSun	1:00u	0	-
731.It Rule	EU	1981	max	\&-	Mar	lastSun	1:00u	1:00	S
732.It Rule	EU	1996	max	\&-	Oct	lastSun	1:00u	0	-
733.El
734.Pp
735.Bl -column -compact -offset indent "# Zone" "Europe/Zurich" "0:29:45.50" "RULES/SAVE" "FORMAT" "UNTIL"
736.It # Zone	NAME	STDOFF	RULES/SAVE	FORMAT	[UNTIL]
737.It Zone	Europe/Zurich	0:34:08	\&-	LMT	1853 Jul 16
738.It 		0:29:45.50	\&-	BMT	1894 Jun
739.It 		1:00	Swiss	CE%sT	1981
740.It 		1:00	EU	CE%sT
741.Pp
742.El
743.Bl -column -compact -offset indent "# Zone" "Europe/Zurich" "0:34:08" "RULES/SAVE" "FORMAT" "UNTIL"
744.It Link	Europe/Zurich	Europe/Vaduz
745.El
746.Pp
747In this example, the timezone is named Europe/Zurich but it has an alias
748as Europe/Vaduz.
749This example says that Zurich was 34 minutes and 8
750seconds east of UT until 1853-07-16 at 00:00, when the legal offset
751was changed to 7\(de\|26\(fm\|22.50\(sd; which this works out to
7520:29:45.50;
753.Nm
754treats this by rounding it to 0:29:46.
755After 1894-06-01 at 00:00 the UT offset became one hour
756and Swiss daylight saving rules (defined with lines beginning with
757.Qq "Rule Swiss"
758apply.
759From 1981 to the present, EU daylight saving rules have
760From 1981 to the present, EU daylight saving rules have
761.Pp
762In 1941 and 1942, daylight saving time applied from the first Monday
763in May at 01:00 to the first Monday in October at 02:00.
764The pre-1981 EU daylight-saving rules have no effect
765here, but are included for completeness.
766Since 1981, daylight
767saving has begun on the last Sunday in March at 01:00 UTC.
768Until 1995 it ended the last Sunday in September at 01:00 UTC,
769but this changed to the last Sunday in October starting in 1996.
770.Pp
771For purposes of display,
772.Qq LMT
773and
774.Qq BMT
775were initially used, respectively.
776Since
777Swiss rules and later EU rules were applied, the time zone abbreviation
778has been CET for standard time and CEST for daylight saving
779time.
780.Sh FILES
781Input files use the format described in this section; output files use
782.Xr tzfile 5
783format.
784.Bl -tag -width /usr/share/zoneinfo -compact
785.It Pa /etc/localtime
786Default local timezone file
787.It Pa /usr/share/zoneinfo
788Default timezone information directory
789.El
790.Sh NOTES
791For areas with more than two types of local time,
792you may need to use local standard time in the
793.Em AT
794field of the earliest transition time's rule to ensure that
795the earliest transition time recorded in the compiled file is correct.
796.Pp
797If,
798for a particular timezone,
799a clock advance caused by the start of daylight saving
800coincides with and is equal to
801a clock retreat caused by a change in UT offset,
802.Ic zic
803produces a single transition to daylight saving at the new UT offset
804without any change in local (wall clock) time.
805To get separate transitions
806use multiple zone continuation lines
807specifying transition instants using universal time.
808.Sh SEE ALSO
809.Xr tzfile 5 ,
810.Xr zdump 8
811.\" @(#)zic.8	8.6
812.\" This file is in the public domain, so clarified as of
813.\" 2009-05-17 by Arthur David Olson.
814