summaryrefslogtreecommitdiff
path: root/doc/getdate.texi
blob: 78522b2ac5abc90a396fee49a7bdd15229177f40 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
@node Date input formats
@chapter Date input formats

@cindex date input formats
@findex getdate

First, a quote:

@quotation
Our units of temporal measurement, from seconds on up to months, are so
complicated, asymmetrical and disjunctive so as to make coherent mental
reckoning in time all but impossible.  Indeed, had some tyrannical god
contrived to enslave our minds to time, to make it all but impossible
for us to escape subjection to sodden routines and unpleasant surprises,
he could hardly have done better than handing down our present system.
It is like a set of trapezoidal building blocks, with no vertical or
horizontal surfaces, like a language in which the simplest thought
demands ornate constructions, useless particles and lengthy
circumlocutions.  Unlike the more successful patterns of language and
science, which enable us to face experience boldly or at least
level-headedly, our system of temporal calculation silently and
persistently encourages our terror of time.

@dots{}  It is as though architects had to measure length in feet, width
in meters and height in ells; as though basic instruction manuals
demanded a knowledge of five different languages.  It is no wonder then
that we often look into our own immediate past or future, last Tuesday
or a week from Sunday, with feelings of helpless confusion.  @dots{}

--- Robert Grudin, @cite{Time and the Art of Living}.
@end quotation

This section describes the textual date representations that GNU
programs accept.  These are the strings you, as a user, can supply as
arguments to the various programs.  The C interface (via the
@code{getdate} function) is not described here.

@cindex beginning of time, for Unix
@cindex epoch, for Unix
Although the date syntax here can represent any possible time since zero
A.D., computer integers are not big enough for such a (comparatively)
long time.  The earliest date semantically allowed on Unix systems is
midnight, 1 January 1970 UCT.

@menu
* General date syntax::            Common rules.
* Calendar date items::             19 Dec 1994.
* Time of day items::               9:20pm.
* Time zone items::                 EST, DST, BST, UTC, ...
* Day of week items::               Monday and others.
* Relative items in date strings::  next tuesday, 2 years ago.
* Pure numbers in date strings::   19931219, 1440.
* Authors of getdate::             Bellovin, Salz, Berets, et al.
@end menu


@node General date syntax
@section General date syntax

@cindex general date syntax

@cindex items in date strings
A @dfn{date} is a string, possibly empty, containing many items
separated by whitespace.  The whitespace may be omitted when no
ambiguity arises.  The empty string means the beginning of today (i.e.,
midnight).  Order of the items is immaterial.  A date string may contain
many flavors of items:

@itemize @bullet
@item calendar date items
@item time of the day items
@item time zone items
@item day of the week items
@item relative items
@item pure numbers.
@end itemize

@noindent We describe each of these item types in turn, below.

@cindex numbers, written-out
@cindex ordinal numbers
@findex first @r{in date strings}
@findex next @r{in date strings}
@findex last @r{in date strings}
A few numbers may be written out in words in most contexts.  This is
most useful for specifying day of the week items or relative items (see
below).  Here is the list: @samp{first} for 1, @samp{next} for 2,
@samp{third} for 3, @samp{fourth} for 4, @samp{fifth} for 5,
@samp{sixth} for 6, @samp{seventh} for 7, @samp{eighth} for 8,
@samp{ninth} for 9, @samp{tenth} for 10, @samp{eleventh} for 11 and
@samp{twelfth} for 12.  Also, @samp{last} means exactly @math{-1}.

@cindex months, written-out
When a month is written this way, it is still considered to be written
numerically, instead of being ``spelled in full''; this changes the
allowed strings.

@cindex case, ignored in dates
@cindex comments, in dates
Alphabetic case is completely ignored in dates.  Comments may be introduced
between round parentheses, as long as included parentheses are properly
nested.  Hyphens not followed by a digit are currently ignored.  Leading
zeros on numbers are ignored.


@node Calendar date items
@section Calendar date items

@cindex calendar date item

A @dfn{calendar date item} specifies a day of the year.  It is
specified differently, depending on whether the month is specified
numerically or literally.  All these strings specify the same calendar date:

@example
1972-09-24     # ISO 8601.
72-9-24        # Assume 19xx for 69 through 99,
               # 20xx for 00 through 68.
72-09-24       # Leading zeros are ignored.
9/24/72        # Common U.S. writing.
24 September 1972
24 Sept 72     # September has a special abbreviation.
24 Sep 72      # Three-letter abbreviations always allowed.
Sep 24, 1972
24-sep-72
24sep72
@end example

The year can also be omitted.  In this case, the last specified year is
used, or the current year if none.  For example:

@example
9/24
sep 24
@end example

Here are the rules.

@cindex ISO 8601 date format
@cindex date format, ISO 8601
For numeric months, the ISO 8601 format
@samp{@var{year}-@var{month}-@var{day}} is allowed, where @var{year} is
any positive number, @var{month} is a number between 01 and 12, and
@var{day} is a number between 01 and 31.  A leading zero must be present
if a number is less than ten.  If @var{year} is 68 or smaller, then 2000
is added to it; otherwise, if @var{year} is less than 100,
then 1900 is added to it.  The construct
@samp{@var{month}/@var{day}/@var{year}}, popular in the United States,
is accepted.  Also @samp{@var{month}/@var{day}}, omitting the year.

@cindex month names in date strings
@cindex abbreviations for months
Literal months may be spelled out in full: @samp{January},
@samp{February}, @samp{March}, @samp{April}, @samp{May}, @samp{June},
@samp{July}, @samp{August}, @samp{September}, @samp{October},
@samp{November} or @samp{December}.  Literal months may be abbreviated
to their first three letters, possibly followed by an abbreviating dot.
It is also permitted to write @samp{Sept} instead of @samp{September}.

When months are written literally, the calendar date may be given as any
of the following:

@example
@var{day} @var{month} @var{year}
@var{day} @var{month}
@var{month} @var{day} @var{year}
@var{day}-@var{month}-@var{year}
@end example

Or, omitting the year:

@example
@var{month} @var{day}
@end example


@node Time of day items
@section Time of day items

@cindex time of day item

A @dfn{time of day item} in date strings specifies the time on a given
day.  Here are some examples, all of which represent the same time:

@example
20:02:0
20:02
8:02pm
20:02-0500      # In EST (Eastern U.S. Standard Time).
@end example

More generally, the time of the day may be given as
@samp{@var{hour}:@var{minute}:@var{second}}, where @var{hour} is
a number between 0 and 23, @var{minute} is a number between 0 and
59, and @var{second} is a number between 0 and 59.  Alternatively,
@samp{:@var{second}} can be omitted, in which case it is taken to
be zero.

@findex am @r{in date strings}
@findex pm @r{in date strings}
@findex midnight @r{in date strings}
@findex noon @r{in date strings}
If the time is followed by @samp{am} or @samp{pm} (or @samp{a.m.}
or @samp{p.m.}), @var{hour} is restricted to run from 1 to 12, and
@samp{:@var{minute}} may be omitted (taken to be zero).  @samp{am}
indicates the first half of the day, @samp{pm} indicates the second
half of the day.  In this notation, 12 is the predecessor of 1:
midnight is @samp{12am} while noon is @samp{12pm}.

@cindex time zone correction
@cindex minutes, time zone correction by
The time may alternatively be followed by a time zone correction,
expressed as @samp{@var{s}@var{hh}@var{mm}}, where @var{s} is @samp{+}
or @samp{-}, @var{hh} is a number of zone hours and @var{mm} is a number
of zone minutes.  When a time zone correction is given this way, it
forces interpretation of the time relative to
Coordinated Universal Time (UTC), overriding any previous
specification for the time zone or the local time zone.  The @var{minute}
part of the time of the day may not be elided when a time zone correction
is used.  This is the only way to specify a time zone correction by
fractional parts of an hour.

Either @samp{am}/@samp{pm} or a time zone correction may be specified,
but not both.


@node Time zone items
@section Time zone items

@cindex time zone item

A @dfn{time zone item} specifies an international time zone, indicated by
a small set of letters.  They are supported for backward compatibility reasons,
but they are not recommended because they are ambiguous in practice:
for example, the abbreviation @samp{EST} has different meanings in
Australia and the United States.  Any included period is ignored.  Military
time zone designations use a single letter.  Currently, only integral
zone hours may be represented in a time zone item.  See the previous
section for a finer control over the time zone correction.

Here are many non-daylight-saving-time time zones, indexed by the zone
hour value.

@table @asis
@item -1200
@samp{Y} for militaries.
@item -1100
@samp{X} for militaries.
@item -1000
@samp{W} for militaries.
@item -0900
@samp{V} for militaries.
@item -0800
@samp{PST} for Pacific Standard, and
@samp{U} for militaries.
@item -0700
@samp{MST} for Mountain Standard, and
@samp{T} for militaries.
@item -0600
@samp{CST} for Central Standard, and
@samp{S} for militaries.
@item -0500
@samp{EST} for Eastern Standard, and
@samp{R} for militaries.
@item -0400
@samp{AST} for Atlantic Standard, and
@samp{Q} for militaries.
@item -0300
@samp{P} for militaries.
@item -0200
@samp{O} for militaries.
@item -0100
@samp{N} for militaries.
@item +0000
@cindex Greenwich Mean Time
@cindex Coordinated Universal Time
@cindex Universal Coordinated Time
@cindex Universal Time (Coordinated)
@samp{GMT} for Greenwich Mean,
@samp{UT} for Universal,
@samp{UTC} for Coordinated Universal,
@samp{WET} for Western European, and
@samp{Z} for ISO 8601 and militaries.
@item +0100
@samp{A} for militaries,
@samp{CET} for Central European,
@samp{MET} for Midden Europesche Tijd (Dutch), and
@samp{MEZ} for Mittel-Europ@"aische Zeit (German).
@item +0200
@samp{B} for militaries, and
@samp{EET} for Eastern European.
@item +0300
@samp{C} for militaries.
@item +0400
@samp{D} for militaries.
@item +0500
@samp{E} for militaries.
@item +0600
@samp{F} for militaries.
@item +0700
@samp{G} for militaries.
@item +0800
@samp{H} for militaries.
@item +0900
@samp{I} for militaries, and
@samp{JST} for Japan Standard.
@item +1000
@samp{GST} for Guam Standard, and
@samp{K} for militaries.
@item +1100
@samp{L} for militaries.
@item +1200
@samp{M} for militaries, and
@samp{NZST} for New Zealand Standard.
@end table

@cindex daylight-saving time
Here are many daylight-saving time (DST) time zones,
indexed by the zone hour value.  Also, by
following a non-DST time zone by the string @samp{DST} in a separate word
(that is, separated by some whitespace), the corresponding DST time zone
may be specified.

@table @asis
@item -0700
@samp{PDT} for Pacific Daylight.
@item -0600
@samp{MDT} for Mountain Daylight.
@item -0500
@samp{CDT} for Central Daylight.
@item -0400
@samp{EDT} for Eastern Daylight.
@item -0300
@samp{ADT} for Atlantic Daylight.
@item +0100
@samp{BST} for British Summer, and
@samp{WEST} for Western European Summer.
@item +0200
@samp{CEST} for Central European Summer,
@samp{MEST} for Midden Europesche S. Tijd (Dutch), and
@samp{MESZ} for Mittel-Europ@"aische Sommerzeit (German).
@item +1300
@samp{NZDT} for New Zealand Daylight.
@end table


@node Day of week items
@section Day of week items

@cindex day of week item

The explicit mention of a day of the week will forward the date
(only if necessary) to reach that day of the week in the future.

Days of the week may be spelled out in full: @samp{Sunday},
@samp{Monday}, @samp{Tuesday}, @samp{Wednesday}, @samp{Thursday},
@samp{Friday} or @samp{Saturday}.  Days may be abbreviated to their
first three letters, optionally followed by a period.  The special
abbreviations @samp{Tues} for @samp{Tuesday}, @samp{Wednes} for
@samp{Wednesday} and @samp{Thur} or @samp{Thurs} for @samp{Thursday} are
also allowed.

@findex next @var{day}
@findex last @var{day}
A number may precede a day of the week item to move forward
supplementary weeks.  It is best used in expression like @samp{third
monday}.  In this context, @samp{last @var{day}} or @samp{next
@var{day}} is also acceptable; they move one week before or after
the day that @var{day} by itself would represent.

A comma following a day of the week item is ignored.


@node Relative items in date strings
@section Relative items in date strings

@cindex relative items in date strings
@cindex displacement of dates

@dfn{Relative items} adjust a date (or the current date if none) forward
or backward.  The effects of relative items accumulate.  Here are some
examples:

@example
1 year
1 year ago
3 years
2 days
@end example

@findex year @r{in date strings}
@findex month @r{in date strings}
@findex fortnight @r{in date strings}
@findex week @r{in date strings}
@findex day @r{in date strings}
@findex hour @r{in date strings}
@findex minute @r{in date strings}
The unit of time displacement may be selected by the string @samp{year}
or @samp{month} for moving by whole years or months.  These are fuzzy
units, as years and months are not all of equal duration.  More precise
units are @samp{fortnight} which is worth 14 days, @samp{week} worth 7
days, @samp{day} worth 24 hours, @samp{hour} worth 60 minutes,
@samp{minute} or @samp{min} worth 60 seconds, and @samp{second} or
@samp{sec} worth one second.  An @samp{s} suffix on these units is
accepted and ignored.

@findex ago @r{in date strings}
The unit of time may be preceded by a multiplier, given as an optionally
signed number.  Unsigned numbers are taken as positively signed.  No
number at all implies 1 for a multiplier.  Following a relative item by
the string @samp{ago} is equivalent to preceding the unit by a
multiplicator with value @math{-1}.

@findex day @r{in date strings}
@findex tomorrow @r{in date strings}
@findex yesterday @r{in date strings}
The string @samp{tomorrow} is worth one day in the future (equivalent
to @samp{day}), the string @samp{yesterday} is worth
one day in the past (equivalent to @samp{day ago}).

@findex now @r{in date strings}
@findex today @r{in date strings}
@findex this @r{in date strings}
The strings @samp{now} or @samp{today} are relative items corresponding
to zero-valued time displacement, these strings come from the fact
a zero-valued time displacement represents the current time when not
otherwise changed by previous items.  They may be used to stress other
items, like in @samp{12:00 today}.  The string @samp{this} also has
the meaning of a zero-valued time displacement, but is preferred in
date strings like @samp{this thursday}.

When a relative item causes the resulting date to cross the boundary
between DST and non-DST (or vice-versa), the hour is adjusted according
to the local time.


@node Pure numbers in date strings
@section Pure numbers in date strings

@cindex pure numbers in date strings

The precise intepretation of a pure decimal number depends
the context in the date string.

If the decimal number is of the form @var{yyyy}@var{mm}@var{dd} and no
other calendar date item (@pxref{Calendar date items}) appears before it
in the date string, then @var{yyyy} is read as the year, @var{mm} as the
month number and @var{dd} as the day of the month, for the specified
calendar date.

If the decimal number is of the form @var{hh}@var{mm} and no other time
of day item appears before it in the date string, then @var{hh} is read
as the hour of the day and @var{mm} as the minute of the hour, for the
specified time of the day.  @var{mm} can also be omitted.

If both a calendar date and a time of day appear to the left of a number
in the date string, but no relative item, then the number overrides the
year.


@node Authors of getdate
@section Authors of @code{getdate}

@cindex authors of @code{getdate}

@cindex Bellovin, Steven M.
@cindex Salz, Rich
@cindex Berets, Jim
@cindex MacKenzie, David
@cindex Meyering, Jim
@code{getdate} was originally implemented by Steven M. Bellovin
(@email{smb@@research.att.com}) while at the University of North Carolina
at Chapel Hill.  The code was later tweaked by a couple of people on
Usenet, then completely overhauled by Rich $alz (@email{rsalz@@bbn.com})
and Jim Berets (@email{jberets@@bbn.com}) in August, 1990.  Various
revisions for the GNU system were made by David MacKenzie, Jim Meyering,
and others.

@cindex Pinard, F.
@cindex Berry, K.
This chapter was originally produced by Fran@,{c}ois Pinard
(@email{pinard@@iro.umontreal.ca}) from the @file{getdate.y} source code,
and then edited by K.@: Berry (@email{kb@@cs.umb.edu}).