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Document zic "out-of-range" times and years

(Problem reported by Michael Deckers.)
* NEWS: Mention this.
* zic.8: Describe longstanding zic behavior for out-of-normal
range years and hours.  Reformat examples to use shorter
lines so that they fit better into man pages.  Use examples
that are a bit closer to the real thing.  Fix typo that
put Zurich west of Greenwich.
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eggert committed Dec 11, 2017
1 parent 017c6ba commit c2cd48af0d764ba7307c5ea7b04cff85e0af1792
Showing with 32 additions and 24 deletions.
  1. +5 −0 NEWS
  2. +27 −24 zic.8
5 NEWS
@@ -52,6 +52,11 @@ Unreleased, experimental changes

Changes to documentation and commentary

The zic man page now documents the longstanding behavior that
times and years can be out of the usual range, with negative times
counting backwards from midnight and with year 0 preceding year 1.
(Problem reported by Michael Deckers.)

The theory.html file now mentions the POSIX limit of six chars
per abbreviation, and lists alphabetic abbreviations used.

51 zic.8
@@ -178,13 +178,14 @@ Gives the (arbitrary) name of the set of rules this rule is part of.
.TP
.B FROM
Gives the first year in which the rule applies.
Any integer year can be supplied; the proleptic Gregorian calendar is assumed.
Any signed integer year can be supplied; the proleptic Gregorian calendar
is assumed, with year 0 preceding year 1.
The word
.B minimum
(or an abbreviation) means the minimum year representable as an integer.
(or an abbreviation) means the indefinite past.
The word
.B maximum
(or an abbreviation) means the maximum year representable as an integer.
(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.
@@ -246,11 +247,13 @@ Recognized forms include:
.nf
.in +.5i
.sp
.ta \w'1:28:13\0\0'u
.ta \w'01:28:13\0\0'u
2 time in hours
2:00 time in hours and minutes
01:28:14 time in hours, minutes, and seconds
15:00 24-hour format time (for times after noon)
1:28:14 time in hours, minutes, and seconds
260:00 260 hours after 00:00
\*-2:30 2.5 hours before 00:00
\*- equivalent to 0
.fi
.in -.5i
@@ -286,10 +289,10 @@ This field has the same format as the
.B AT
field
(although, of course, the
.B w
and
.B s
suffixes are not used).
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.
Only the sum of standard time and this amount matters; for example,
.I zic
does not distinguish a 10:30 standard time plus an 0:30
@@ -317,13 +320,13 @@ A zone line has the form
.sp
.nf
.ti +.5i
.ta \w'Zone\0\0'u +\w'Australia/Adelaide\0\0'u +\w'GMTOFF\0\0'u +\w'RULES/SAVE\0\0'u +\w'FORMAT\0\0'u
Zone NAME GMTOFF RULES/SAVE FORMAT [UNTILYEAR [MONTH [DAY [TIME]]]]
.ta \w'Zone\0\0'u +\w'Asia/Amman\0\0'u +\w'GMTOFF\0\0'u +\w'Jordan\0\0'u +\w'FORMAT\0\0'u
Zone NAME GMTOFF RULES FORMAT [UNTIL]
.sp
For example:
.sp
.ti +.5i
Zone Australia/Adelaide 9:30 Aus AC%sT 1971 Oct 31 2:00
Zone Asia/Amman 2:00 Jordan EE%sT 2017 Oct 27 01:00
.sp
.fi
The fields that make up a zone line are:
@@ -348,8 +351,8 @@ and
fields of rule lines;
begin the field with a minus sign if time must be subtracted from UT.
.TP
.B RULES/SAVE
The name of the rule(s) that apply in the time zone or,
.B RULES
The name of the rules that apply in the time zone or,
alternately, an amount of time to add to local standard time.
If this field is
.B \*-
@@ -383,9 +386,9 @@ separates standard and daylight abbreviations.
To conform to POSIX, a time zone abbreviation should contain only
alphanumeric ASCII characters, "+" and "\*-".
.TP
.B UNTILYEAR [MONTH [DAY [TIME]]]
.B UNTIL
The time at which the UT offset or the rule(s) change for a location.
It is specified as a year, a month, a day, and a time of day.
It takes the form of 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
@@ -454,7 +457,7 @@ Leap YEAR MONTH DAY HH:MM:SS CORR R/S
For example:
.ti +.5i
.sp
Leap 1974 Dec 31 23:59:60 + S
Leap 2016 Dec 31 23:59:60 + S
.sp
.fi
The
@@ -507,25 +510,25 @@ 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 \*-
.sp
.ta \w'# Zone\0\0'u +\w'Europe/Zurich\0\0'u +\w'GMTOFF\0\0'u +\w'RULES/SAVE\0\0'u +\w'FORMAT\0\0'u
# Zone NAME GMTOFF RULES/SAVE FORMAT UNTIL
.ta \w'# Zone\0\0'u +\w'Europe/Zurich\0\0'u +\w'GMTOFF\0\0'u +\w'0:34:08\0\0'u +\w'FORMAT\0\0'u
# Zone NAME GMTOFF RULES FORMAT [UNTIL]
Zone Europe/Zurich 0:34:08 \*- LMT 1853 Jul 16
0:29:46 \*- BMT 1894 Jun
1:00 Swiss CE%sT 1981
1:00 EU CE%sT
.sp
Link Europe/Zurich Switzerland
Link Europe/Zurich Europe/Vaduz
.sp
.in
.fi
In this example, the zone is named Europe/Zurich but it has an alias
as Switzerland. This example says that Zurich was 34 minutes and 8
seconds west of UT until 1853-07-16 at 00:00, when the legal offset
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\(de\|26\(fm\|22.50\(sd; although this works out to
0:29:45.50, the input format cannot represent fractional seconds so it
is rounded here. After 1894-06-01 at 00:00 Swiss daylight saving rules
(defined with lines beginning with "Rule Swiss") apply, and the UT offset
became one hour. From 1981 to the present, EU daylight saving rules have
is rounded here. 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.
.PP
In 1941 and 1942, daylight saving time applied from the first Monday

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