1Date::Manip::Recur(3) User Contributed Perl DocumentationDate::Manip::Recur(3)
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6 Date::Manip::Recur - methods for working with recurring events
7
9 use Date::Manip::Recur;
10 $date = new Date::Manip::Recur;
11
13 This module contains functions useful in parsing and manipulating
14 recurrences. A recurrence is a notation for specifying when a
15 recurring event occurs. For example, if an event occurs every other
16 Friday or every 4 hours, this can be defined as a recurrence. A fully
17 specified recurrence consists of the following pieces of information:
18 Frequency
20 The most basic piece of information is the frequency. For
21 relatively simple recurring events, the frequency defines when
22 those events occur. For more complicated recurring events, the
23 frequency tells approximately when the events occur (but to get the
24 actual events, the modifiers must be applied as described below).
25 Examples include:
27 the first of every month
29 every other day
30 the 4th Thursday of each month at 2:00 PM
31 every 2 hours and 30 minutes
32 All of these can be expressed as a frequency.
34 NOTE: unlike date parsing, support for frequencies written out in
36 English (or whatever language you are working in) is extremely
37 limited. For example, the string "the first of every month" will
38 NOT be parsed as a valid frequency. A limited number of frequencies
39 can be expressed in a written out form (see OTHER FREQUENCY FORMATS
40 below), but most must be expressed in the format described below in
41 FREQUENCY NOTATION. In the documentation however, the written out
42 form will often be used for the sake of clarity.
43 Since a frequency typically refers to events could happen an
45 infinite number of times, you must specify either a date range or a
46 base date (or both) in order to determine actual dates on which an
47 event occurred.
48 Modifier
50 Complex recurring events may require the use of modifiers in order
51 to get them correct.
52 For example, in America many places treat both Thanksgiving and the
54 day after as holidays. Thanksgiving is easy to define as the
55 frequency:
56 4th Thursday of every November
58 but the day after is NOT possible to define only as a frequency.
60 Depending on the year, the day after the 4th Thursday may be the
61 4th or 5th Friday.
62 The day after Thanksgiving must be defined as a frequency and a
64 modifier:
65 4th Thursday of every November
67 +1 day
68 The syntax for the various modifiers is described below in the
70 MODIFIERS section.
71 Base date
73 All recurrences have a base date which is a date on which a
74 recurring event is based.
75 The base date is not necessarily a date where the recurring event
77 occurs. Instead, it may be modified (with modifiers, or with values
78 specified in the recurrence) to actually produce a recurring event.
79 For example, if the frequency is
81 every other Friday at noon
83 the base date will be a Friday and the recurring event will happen
85 on that Friday, Friday two weeks later, Friday four weeks later,
86 etc. In all cases, the dates will be modified to be at noon.
87 If the frequency has a modifier, such as:
89 every other Friday
91 + 1 day
92 (and yes, this trivial example could be expressed as the frequency
94 'every other Saturday' with no modifiers), then the base date is
95 still on a Friday, but the actual recurring event is determined by
96 applying modifiers and occurs on Saturday.
97 Recurring events are assigned a number with the event that is
99 referred to by the base date being the 0th occurrence, the first
100 one after that as the 1st occurrence, etc. Recurring events can
101 also occur before the base date with the last time the recurring
102 event occurred before the base date is the -1th occurence.
103 So, if the frequency is
105 the first of every month
107 and the base date is 'Mar 1, 2000', then the 5 recurring events
109 around it are:
110 N Date
112 -2 Jan 1 2000
114 -1 Feb 1 2000
115 0 Mar 1 2000
116 +1 Apr 1 2000
117 +2 May 1 2000
118 In some cases, the Nth date may not be defined. For example, if the
120 frequency is:
121 the 31st of every month
123 and the base date is Mar 31, 2000, the 5 recurring events around it
125 are:
126 N Date
128 -2 Jan 31 2000
130 -1 undefined
131 0 Mar 31 2000
132 1 undefined
133 2 May 31 2000
134 There are actually two different uses of the term 'base date' which
136 should be understood.
137 The first is the base date as specified by the programmer. The
139 second is the actual base date as used internally by Date::Manip.
140 The information in the base date specified by the programmer may
142 not actually refer to a date which matches the frequency. In this
143 case, the base date used by Date::Manip is based on the specified
144 base date, but is not identical to it. For more details, refer to
145 the BASE DATES section below.
146 For example, with the frequence 'the first of every month', if the
148 specified base date is 'Mar 1, 2000', this refers to a date on
149 which the recurring event occurred, so the actual base date is the
150 same as the specified one.
151 If the specified base date were 'Feb 20, 2000', the actual base
153 date would be use the month (Feb) of the specified base date, but
154 not the day, and the actual base date would be 'Feb 1, 2000'.
155 Range
157 A date range is simply a starting and an ending date. When a range
158 is used (primarily in the dates method as described below), only
159 recurring events (with all modifiers applied) which happened on or
160 after the start date and on or before the end date are used.
161 For example, if the frequency was
163 the first of every month
165 and the start/end dates were Jan 1 2000 and May 31 2000, the list
167 of dates referred to would be:
168 Jan 1 2000
170 Feb 1 2000
171 Mar 1 2000
172 Apr 1 2000
173 May 1 2000
174 If no base date is specified, but a date range is specified, the
176 start date is used as the specified base date.
177 It should be noted that if both the range and base date are
179 specified, the range is not used to determine a base date. Also,
180 the first time the recurring event occurs in this range may NOT be
181 the 0th occurrence with respect to the base date, and that is
182 allowed.
183 NOTE: both dates in the range and the base date must all be in the
185 same time zone, and use the same Date::Manip::Base object.
186
188 The syntax for specifying a frequency requires some explanation. It is
189 very concise, but contains the flexibility to express every single type
190 of recurring event I could think of.
191 The syntax of the frequency description is a colon separated list of
193 the format Y:M:W:D:H:MN:S (which stand for year, month, week, etc.).
194 One (and only one) of the colons may optionally be replaced by an
195 asterisk, or an asterisk may be prepended to the string. For example,
196 the following are all valid frequency descriptions:
197 1:2:3:4:5:6:7
199 1:2*3:4:5:6:7
200 *1:2:3:4:5:6:7
201 But the following are NOT valid because they contain more than one
203 asterisk:
204 1:2*3:4:5*6:7
206 *1:2:3:4:5:6*7
207 When an asterisk is included, the portion to the left of it is called
209 the interval, and refers to an approximate time interval between
210 recurring events. For example, if the interval of the frequency is:
211 1:2*
213 it means that the recurring event occurs approximately every 1 year and
215 2 months. The interval is approximate because elements to the right of
216 the asterisk, as well as any modifiers included in the recurrence, will
217 affect when the events actually occur.
218 If no asterisks are included, then the entire recurrence is an
220 interval. For example,
221 0:0:0:1:12:0:0
223 refers to an event that occurs every 1 day, 12 hours.
225 The portion of the frequency that occur after an asterisk is called the
227 recurrence time (or rtime), and refers to a specific value (or values)
228 for that type of time element (i.e. exactly as it would appear on a
229 calendar or a clock). For example, if the frequency ends with the
230 rtime:
231 *12:0:0
233 then the recurring event occurs at 12:00:00 (noon).
235 For example:
237 0:0:0:2*12:30:0 every 2 days at 12:30 (each day)
239 Elements in the rtime can be listed as single values, ranges (2 numbers
241 separated by a dash "-"), or a comma separated list of values or
242 ranges. In some cases, negative values are appropriate for the week or
243 day values. -1 stands for the last possible value, -2 for the second to
244 the last, etc.
245 If multiple values are included in more than one field in the rtime,
247 every possible combination will be used. For example, if the frequency
248 ends with the rtime:
249 *12-13:0,30:0
251 the event will occur at 12:00, 12:30, 13:00, and 13:30.
253 Some examples are:
255 0:0:0:1*2,4,6:0:0 every day at at 02:00, 04:00, and 06:00
257 0:0:0:2*12-13:0,30:0 every other day at 12:00, 12:30, 13:00,
258 and 13:30
259 0:1:0*-1:0:0:0 the last day of every month
260 *1990-1995:12:0:1:0:0:0
261 Dec 1 in 1990 through 1995
262 There is no way to express the following with a single recurrence:
264 every day at 12:30 and 1:00
266 You have to use two recurrences to do this.
268 You can include negative numbers in ranges. For example, including the
270 range -2---1 means to go from the 2nd to the last to the last
271 occurrence. Negative values are only supported in the week and day
272 fields, and only in some cases.
273 You can even use a range like 2--2 (which means to go from the 2nd to
275 the 2nd to the last occurrence). However, this is STRONGLY discouraged
276 since this leads to a date which produces a variable number of events.
277 As a result, the only way to determine the Nth date is to calculate
278 every date starting at the base date. If you know that every date
279 produces exactly 4 recurring events, you can calculate the Nth date
280 without needing to determine every intermediate date.
281 When specifying a range, the first value must be less than the second
283 or else nothing will be returned.
284 When both the week and day elements are non-zero and the day is right
286 of the asterisk, the day refers to the day of week. The following
287 examples illustrate these type of frequencies:
288 0:1*4:2:0:0:0 4th Tuesday (day 2) of every month
290 0:1*-1:2:0:0:0 last Tuesday of every month
291 0:0:3*2:0:0:0 every 3rd Tuesday (every 3 weeks
292 on 2nd day of week)
293 1:0*12:2:0:0:0 the 12th Tuesday of each year
294 NOTE: The day of week refers to the numeric value of each day as
296 specified by ISO 8601. In other words, day 1 is ALWAY Monday, day 7 is
297 ALWAYS Sunday, etc., regardless of what day of the week the week is
298 defined to begin on (using the FirstDay config variable). So when the
299 day field refers to the day of week, it's value (or values if a range
300 or comma separated list are used) must be 1-7.
301 When the week element is zero and the month element is non-zero and the
303 day element is right of the asterisk, the day value is the day of the
304 month (it can be from 1 to 31 or -1 to -31 counting from the end of the
305 month).
306 3*1:0:2:12:0:0 every 3 years on Jan 2 at noon
308 0:1*0:2:12,14:0:0 2nd of every month at 12:00 and 14:00
309 0:1:0*-2:0:0:0 2nd to last day of every month
310 NOTE: If the day given refers to the 29th, 30th, or 31st, in a month
312 that does not have that number of days, it is ignored. For example, if
313 you ask for the 31st of every month, it will return dates in Jan, Mar,
314 May, Jul, etc. Months with fewer than 31 days will be ignored.
315 If both the month and week elements are zero, and the year element is
317 non-zero, the day value is the day of the year (1 to 365 or 366 -- or
318 the negative numbers to count backwards from the end of the year).
319 1:0:0*45:0:0:0 45th day of every year
321 Specifying a day that doesn't occur in that year silently ignores that
323 year. The only result of this is that specifying +366 or -366 will
324 ignore all years except leap years.
325 If the week element is non-zero and to the right of the asterisk, and
327 the day element is zero, the frequency refers to the first day of the
328 given week of the month or week of the year:
329 0:1*2:0:0:0:0 the first day of the 2nd week of
331 every month
332 1:0*2:0:0:0:0 the first day of the 2nd week of
333 every year
334 Although the meaning of almost every recurrence can be deduced by the
336 above rules, a set of tables describing every possible combination of
337 Y/M/W/D meanings, and giving an example of each is included below in
338 the section LIST OF Y/M/W/D FREQUENCY DEFINITIONS. It also explains a
339 small number of special cases.
340 NOTE: If all fields left of the asterisk are zero, the last one is
342 implied to be 1. In other words, the following are equivalent:
343 0:0:0*x:x:x:x
345 0:0:1*x:x:x:x
346 and can be thought of as every possible occurence of the rtime.
348 NOTE: When applying a frequency to get a list of dates on which a
350 recurring event occurs, a delta is created from the frequency which is
351 applied to get dates referred to by the interval. These are then
352 operated on by the rtime and by modifiers to actually get the recurring
353 events. The deltas will always be exact or approximate. There is no
354 support for business mode recurrences. However, with the careful use of
355 modifiers (discussed below), most recurring business events can be
356 determined too.
357
359 A recurrence of the form *Y:M:W:D:H:MN:S (which is technically speaking
360 not a recurring event... it is just a date or dates specified using the
361 recurrence syntax) uses the first date which matches the frequency as
362 the base date. Any base date specified will be completely ignored. A
363 date range may be specified to work with a subset of the dates.
364 All other recurrences use a specified base date in order to determine
366 when the 0th occurrence of a recurring event happens. As mentioned
367 above, the specified base date may be determined from the start date,
368 or specified explicitly.
369 The specified base date is used to provide the bare minimum
371 information. For example, the recurrence:
372 0:0:3*4:0:0:0 every 3 weeks on Thursday
374 requires a base date to determine the week, but nothing else. Using the
376 standard definition (Monday-Sunday) for a week, and given that one week
377 in August 2009 is Aug 10 to Aug 16, any date in the range Aug 10 to Aug
378 16 will give the same results. The definition of the week defaults to
379 Monday-Sunday, but may be modified using the FirstDay config variable.
380 Likewise, the recurrence:
382 1:3*0:4:0:0:0 every 1 year, 3 months on the 4th
384 day of the month
385 would only use the year and month of the base date, so all dates in a
387 given month would give the same set of recurring dates.
388 It should also be noted that a date may actually produce multiple
390 recurring events. For example, the recurrence:
391 0:0:2*4:12,14:0:0 every 2 weeks on Thursday at 12:00
393 and 14:00
394 produces 2 events for every date. So in this case, the base date
396 produces the 0th and 1st event, the base date + an offset produces the
397 2nd and 3rd events, etc.
398 It must be noted that the base date refers ONLY to the interval part of
400 the recurrence. The rtime and modifiers are NOT used in determining the
401 base date.
402
404 The interval of a frequency (everything left of the asterisk) will be
405 used to generate a list of dates (called interval dates). When rtime
406 values and modifiers are applied to an interval date, it produces the
407 actual recurring events.
408 As already noted, if the rtime values include multiple values for any
410 field, more than one event are produced by a single interval date.
411 It is important to understand is how the interval dates are calculated.
413 The interval is trivially turned into a delta. For example, with the
414 frequency 0:0:2*4:12:0:0, the interval is 0:0:2 which produces the
415 delta 0:0:2:0:0:0:0.
416 In order to get the Nth interval date, the delta is multiplied by N and
418 added to the base date. In other words:
419 D(0) = Jan 31
421 D(1) = Jan 31 + 1 month = Feb 28
422 D(2) = Jan 31 + 2 month = Mar 31
423
425 The start and end dates form the range in which recurring events can
426 fall into.
427 Every recurring date will fall in the limit:
429 start <= date <= end
431 When a recurrence is created, it may include a default range, and this
433 is handled by the RecurRange config variable.
434
436 There are a small handful of English strings (or the equivalent in
437 other languages) which can be parsed in place of a numerical frequency.
438 These include:
439 every Tuesday in June [1997]
441 2nd Tuesday in June [1997]
442 last Tuesday in June [1997]
443 every Tuesday of every month [in 1997]
445 2nd Tuesday of every month [in 1997]
446 last Tuesday of every month [in 1997]
447 every day of every month [in 1997]
449 2nd day of every month [in 1997]
450 last day of every month [in 1997]
451 every day [in 1997]
453 every 2nd day [in 1977]
454 every 2 days [in 1977]
455 Each of these set the frequency. If the year is include in the string,
457 it also sets the dates in the range to be the first and last day of the
458 year.
459 In each of these, the numerical part (i.e. 2nd in all of the examples
461 above) can be any number from 1 to 31. To make a frequency with a
462 larger number than that, you have to use the standard format discussed
463 above.
464 Due to the complexity of writing out (and parsing) frequencies written
466 out, I do not intend to add additional frequency formats, and the use
467 of these is discouraged. The frequency format described above is
468 preferred.
469
471 Any number of modifiers may be added to a frequency to get the actual
472 date of a recurring event. Modifiers are case sensitive.
473 Modifiers to set the day-of-week
475 The following modifiers can be used to adjust a date to a specific
476 day of the week.
477 PDn Means the previous day n not counting today
479 PTn Means the previous day n counting today
480 NDn Means the next day n not counting today
481 NTn Means the next day n counting today
482 In each of these, 'n' is 1-7 (1 being Sunday, 7 being Saturday).
484 For example, PD2/ND2 returns the previous/next Tuesday. If the date
486 that this is applied to is Tuesday, it modifies it to one week in
487 the past/future.
488 PT2/NT2 are similar, but will leave the date unmodified if it is a
490 Tuesday.
491 Modifiers to move forward/backward a number of days
493 These modifiers can be used to add/subtract n days to a date.
494 FDn Means step forward n days.
496 BDn Means step backward n days.
497 Modifiers to force events to be on business days
499 Modifiers can also be used to force recurring events to occur on
500 business days. These modifiers include:
501 FWn Means step forward n workdays.
503 BWn Means step backward n workdays.
504 CWD The closest work day (using the TomorrowFirst
506 config variable).
507 CWN The closest work day (looking forward first).
508 CWP The closest work day (looking backward first).
509 NWD The next work day counting today
511 PWD The previous work day counting today
512 DWD The closest work day (using the TomorrowFirst config
513 variable) counting today
514 The CWD, CWN, and CWP modifiers will always change the date to the
516 closest working day NOT counting the current date.
517 The NWD, PWD, and DWD modifiers always change the date to the
519 closest working day unless the current date is a work day. In that
520 case, it is left unmodified.
521 CWD, CWN, and CWP will usually return the same value, but if you
523 are starting at the middle day of a 3-day weekend (for example), it
524 will return either the first work day of the following week, or the
525 last work day of the previous week depending on whether it looks
526 forward or backward first.
527 All business day modifiers ignore the time, so if a date is
529 initially calculated at Saturday at noon, and the FW1 is applied,
530 the date is initially moved to the following Monday (assuming it is
531 a work day) and the FW1 moves it to Tuesday. The final result will
532 be Tuesday at noon.
533 Special modifiers
535 The following modifiers do things that cannot be expressed using
536 any other combination of frequency and modifiers:
537 EASTER Set the date to Easter for this year.
539
541 In order to get a list of dates referred to by the recurrence, the
542 following steps are taken.
543 The recurrence is tested for errors
545 The recurrence must be completely specified with a base date
546 (either supplied explicitly, or derived from a start date) and date
547 range when necessary. All dates must be valid.
548 The actual base date is determined
550 Using information from the interval and the specified base date,
551 the actual base date is determined.
552 The Nth date is calculated
554 By applying the delta that corresponds to the interval, and then
555 applying rtime and modifier information, the Nth date is
556 determined.
557 This is repeated until all desired dates have been obtained.
559 The nth method described below has more details.
561 The range is tested
563 Any date that fall outside the range is discarded.
564 NOTE: when the recurrence contains no interval, it is not necessary
566 to specify the range, and if it is not specified, all of the dates
567 are used. The range MAY be specified to return only a subset of the
568 dates if desired.
569
571 Because the week and day values may have multiple meanings depending on
572 where the asterisk is, and which of the fields have non-zero values, a
573 list of every possible combination is included here (though most can be
574 determined using the rules above).
575 When the asterisk occurs before the day element, and the day element is
577 non-zero, the day element can take on multiple meanings depending on
578 where the asterisk occurs, and which leading elements (year, month,
579 week) have non-zero values. It can refer to the day of the week, day of
580 the month, or day of the year.
581 When the asterisk occurs before the week element, the week element of
583 the frequency can also take on multiple meanings as well. When the
584 month field and day fields are zero, it refers to the week of the year.
585 Since the week of the year is well defined in the ISO 8601 spec, there
586 is no ambiguity.
587 When the month field is zero, but the day field is not, the week field
589 refers to the nth occurrence of the day of week referred to by the day
590 field in the year.
591 When the month field is non-zero, the week field refers to the nth
593 occurrence of the day of week in the month.
594 In the tables below only the first 4 elements of the frequency are
596 shown. The actual frequency will include the hour, minute, and second
597 elements in addition to the ones shown.
598 When all elements left of the asterisk are 0, the interval is such that
600 it occurs the maximum times possible (without changing the type of
601 elements to the right of the asterisk). Another way of looking at it is
602 that the last 0 element of the interval is changed to 1. So, the
603 interval:
604 0:0*3:0
606 is equivalent to
608 0:1*3:0
610 When the year field is zero, and is right of the asterisk, it means the
612 current year.
613 All elements left of the asterisk
615 When all of the month, week, and day elements are left of the
616 asterisk, the simple definitions of the frequency are used:
617 frequency meaning
619 1:2:3:4 every 1 year, 2 months, 3 weeks,
621 4 days
622 Any, or all of the fields can be zero.
624 Non-zero day, non-zero week
626 When both the day and week elements are non-zero, the day element
627 always refers to the day of week. Values must be in the range (1 to
628 7) and no negative values are allowed.
629 The following tables shows all possible variations of the frequency
631 where this can happen (where day 4 = Thursday).
632 When the week is left of the asterisk, the interval is used to get
634 the weeks on the calendar containing a recurring date, and the day
635 is used to set the day of the week. The following are possible:
636 frequency meaning
638 1:2:3*4 every 1 year, 2 months, 3 weeks
640 on Thur
641 1:0:3*4 every 1 year, 3 weeks on Thur
643 0:2:3*4 every 2 months, 3 weeks on Thur
645 0:0:3*4 every 3 weeks on Thur
647 When the week is right of the asterisk, and a non-zero month is
649 left of the asterisk, the recurrence refers to a specific
650 occurrence of a day-of-week during a month. The following are
651 possible:
652 frequency meaning
654 1:2*3:4 every 1 year, 2 months on the
656 3rd Thursday of the month
657 0:2*3:4 every 2 months on the 3rd Thur
659 of the month
660 When the week and month are both non-zero and right of the
662 asterisk, the recurrence refers to an occurrence of day-of-week
663 during the given month. Possibilities are:
664 frequency meaning
666 1*2:3:4 every 1 year in February on
668 the 3rd Thur
669 0*2:3:4 same as 1*2:3:4
671 *1:2:3:4 in Feb 0001 on the 3rd Thur
673 of the month
674 *0:2:3:4 on the 3rd Thur of Feb in the
676 current year
677 When the week is right of the asterisk, and the month is zero, the
679 recurrence refers to an occurence of the day-of-week during the
680 year. The following are possible:
681 frequency meaning
683 1:0*3:4 every 1 year on the 3rd Thursday
685 1*0:3:4 of the year
686 *1:0:3:4 in 0001 on the 3rd Thur of
688 the year
689 0*0:3:4 same as 1*0:3:4
691 *0:0:3:4 on the 3rd Thur of the current
693 year
694 There is one special case:
696 frequency meaning
698 0:0*3:4 same as 0:1*3:4 (every month on
700 the 3rd Thur of the month)
701 Non-zero day, non-zero month
703 When a non-zero day element occurs to the right of the asterisk and
704 the week element is zero, but the month element is non-zero, the
705 day elements always refers to a the day of month in the range (1 to
706 31) or (-1 to -31).
707 The following table shows all possible variations of the frequency
709 where this can happen:
710 frequency meaning
712 1:2:0*4 every 1 year, 2 months on the
714 1:2*0:4 4th day of the month
715 1*2:0:4 every year on Feb 4th
717 *1:2:0:4 Feb 4th, 0001
719 0:2:0*4 every 2 months on the 4th day
721 0:2*0:4 of the month
722 0*2:0:4 same as 1*2:0:4
724 *0:2:0:4 Feb 4th of the current year
726 Zero day, non-zero week
728 When a day is zero, and the week is non-zero, the recurrence refers
729 to a specific occurrence of the first day of the week (as given by
730 the FirstDay variable).
731 The frequency can refer to an occurrence of FirstDay in a specific
733 week (if the week is left of the asterisk):
734 frequency meaning
736 1:2:3*0 every 1 year, 2 months, 3 weeks on
738 FirstDay
739 1:0:3*0 every 1 year, 3 weeks on FirstDay
741 0:2:3*0 every 2 months, 3 weeks on FirstDay
743 0:0:3*0 every 3 weeks on FirstDay
745 or to a week in the year (if the week is right of the asterisk, and
747 the month is zero):
748 frequency meaning
750 1:0*3:0 every 1 year on the first day of the
752 1*0:3:0 3rd week of the year
753 *1:0:3:0 the first day of the 3rd week of 0001
755 or to an occurrence of FirstDay in a month (if the week is right of
757 the asterisk and month is non-zero):
758 frequency meaning
760 1:2*3:0 every 1 year, 2 months on the 3rd
762 occurence of FirstDay
763 0:2*3:0 every 2 months on the 3rd occurence
765 of FirstDay
766 1*2:3:0 every year on the 3rd occurence
768 of FirstDay in Feb
769 0*2:3:0 same as 1*2:3:0
771 *1:2:3:0 the 3rd occurence of FirstDay
773 Feb 0001
774 *0:2:3:0 the 3rd occurence of FirstDay
776 in Feb of the current year
777 NOTE: in the last group, a slightly more intuitive definition of
779 these would have been to say that the week field refers to the week
780 of the month, but given the ISO 8601 manner of defining when weeks
781 start, this definition would have virtually no practical
782 application. So the definition of the week field referring to the
783 Nth occurence of FirstDay in a month was used instead.
784 There are a few special cases here:
786 frequency meaning
788 0:0*3:0 same as 0:1*3:0 (every month on the 3rd
790 occurence of the first day of week)
791 0*0:3:0 same as 1*0:3:0
793 *0:0:3:0 the first day of the 3rd week of the
795 current year
796 Non-zero day
798 When a non-zero day element occurs and both the month and week
799 elements are zero, the day elements always refers to a the day of
800 year (1 to 366 or -1 to -366 to count from the end).
801 The following table shows all possible variations of the frequency
803 where this can happen:
804 frequency meaning
806 1:0:0*4 every year on the 4th day of
808 1:0*0:4 the year
809 1*0:0:4
810 *1:0:0:4 the 4th day of 0001
812 Other non-zero day variations have multiple meanings for the day
814 element:
815 frequency meaning
817 0:0:0*4 same as 0:0:1*4 (every week on Thur)
819 0:0*0:4 same as 0:1*0:4 (every month on the 4th)
821 0*0:0:4 same as 1*0:0:4
823 *0:0:0:4 the 4th day of the current year
825 All other variations
827 The remaining variations have zero values for both week and day.
828 They are:
829 frequency meaning
831 1:2:0*0 every 1 year, 2 months on the first
833 1:2*0:0 day of the month
834 1*2:0:0 every year on Feb 1
836 *1:2:0:0 Feb 1, 0001
838 1:0:0*0 every 1 year on Jan 1
840 1:0*0:0
841 1*0:0:0
842 *1:0:0:0 Jan 1, 0001
844 0:2:0*0 every 2 months on the first day of
846 0:2*0:0 the month
847 0*2:0:0 same as 1*2:0:0
849 *0:2:0:0 Feb 1 of the current year
851 0:0:0*0 same as 0:0:1*0 (every week on
853 the first day of the week)
854 0:0*0:0 same as 0:1*0:0 (every month
856 on the 1st)
857 0*0:0:0 same as 1*0:0:0
859 *0:0:0:0 Jan 1 of the current year
861
863 new
864 new_config
865 new_date
866 new_delta
867 new_recur
868 base
869 tz
870 is_date
871 is_delta
872 is_recur
873 config
874 err Please refer to the Date::Manip::Obj documentation for these
875 methods.
876 parse
878 $err = $recur->parse($string [,$modifiers] [,$base,$start,$end]);
879 This creates a new recurrence. A string containing a valid
881 frequency is required. In addition, $start, $end, and $base dates
882 can be passed in (either as Date::Manip::Date objects, or as
883 strings containing dates that can be parsed), and any number of the
884 modifiers listed above.
885 If the $start or $end dates are not included, they may be supplied
887 automatically, based on the value of the RecurRange variable. If
888 any of the dates are passed in, they must be included in the order
889 given (though it is safe to pass an empty string or undef in for
890 any of them if you only want to set some, but not all of them).
891 The $modifiers argument must either contain valid modifiers, or be
893 left out of the argument list entirely. You cannot pass an empty
894 string or undef in for it.
895 $err = $recur->parse($string);
897 This creates a recurrence from a string which contains all of the
899 necessary elements of the recurrence. The string is of the format:
900 FREQ*MODIFIERS*BASE*START*END
902 where FREQ is a string containing a frequency, MODIFIERS is a
904 string containing a comma separated list of modifiers, BASE, START,
905 and END are strings containing parseable dates.
906 All pieces are optional, but order must be maintained, so all of
908 the following are valid:
909 FREQ*MODIFIERS
911 FREQ**BASE
912 FREQ**BASE*START*END
913 If a part of the recurrence is passed in both as part of $string
915 and as an argument, the argument overrides the string portion, with
916 the possible exception of modifiers. The modifiers in the argument
917 override the string version unless the first one is a '+' in which
918 case they are appended. See the modifiers method below for more
919 information.
920 frequency
922 start
923 end
924 basedate
925 modifiers
926 You can also create a recurrency in steps (or replace parts of an
927 existing recurrence) using the following:
928 $err = $recur->frequency($frequency);
930 $err = $recur->start($start);
932 $err = $recur->end($end);
933 $err = $recur->basedate($base);
935 $err = $recur->modifiers($modifiers);
937 $err = $recur->modifiers(@modifiers);
938 These set the appropriate part of the recurrence.
940 Calling the frequency method discards all information currently
942 stored in the Recur object (including an existing start, end, and
943 base date), so this method should be called first.
944 In the modifiers method, the modifiers can be passed in as a string
946 containing a comma separated list of modifiers, or as a list of
947 modifiers. The modifiers passed in override all previously set
948 modifiers UNLESS the first one is the string "+", in which case the
949 new modifiers are appended to the list.
950 In the start, end, and base methods, the date passed in can be a
952 Date::Manip::Date object, or a string that can be parsed to get a
953 date.
954 NOTE: the parse method will overwrite all parts of the recurrence,
956 so it is not appropriate to do:
957 $recur->modifiers($modifiers);
959 $recur->parse($string);
960 The modifiers passed in in the first call will be overwritten.
962 These functions can also be used to look up the values.
964 $freq = $recur->frequency();
966 $start = $recur->start();
967 $end = $recur->end();
968 @mods = $recur->modifiers();
969 ($base,$actual) = $recur->basedate();
971 The basedate function will return both the specified base and the
973 actual base dates.
974 If any of the values are not yet determined, nothing will be
976 returned.
977 dates
979 @dates = $recur->dates([$start,$end]);
980 Returns the list of dates defined by the full recurrence. If there
982 is an error, or if there are no dates, an empty list will be
983 returned.
984 $start and $end are either undef, or dates which can be used to
986 limit the set of dates passed back (they can be Date::Manip::Date
987 objects or strings that can be parsed).
988 If the recurrence does not have a start and end date already,
990 passing in $start and $end will set the range (but they will NOT be
991 stored in the recurrence).
992 If the recurrence does have a start and end date stored in it, the
994 $start and $end arguments can be used to temporarily override the
995 limits. For example, if a recurrence has a start date of Jan 1,
996 2006 00:00:00 and and end date of Dec 31, 2006 23:59:59 stored in
997 the recurrence, passing in $start of Jul 1, 2006 00:00:00 will
998 limit the dates returned to the range of Jul 1 to Dec 31.
999 Passing in a start date of Jul 1, 2007 will mean that no dates are
1001 returned since the recurrence limits the date to be in 2006.
1002 If one or both of $start and $end are undef, then the stored values
1004 will be used.
1005 nth
1007 ($date,$err) = $recur->nth($n);
1008 This returns the $n'th recurring event ($n may be any integer). If
1010 an error occurs, it is returned (but it is not set in $recur since
1011 it may be properly, though perhaps incompletely, defined). The
1012 following errors may be returned:
1013 Invalid recurrence
1015 The recurrence has an error flag set.
1016 Incomplete recurrence
1018 The recurrence is incomplete. It needs either a
1019 base date or a date range.
1020 Range invalid
1022 The recurrence has an invalid date range (i.e.
1023 the end date occurs before the start date).
1024 Start invalid
1026 End invalid
1027 Base invalid
1028 An invalid date was entered for one of the dates.
1029 There are a few special circumstances to be aware of.
1031 1) If the recurrence contains no interval (i.e. is of the form
1033 *Y:M:W:D:H:MN:S), the dates come directly from the rtime values.
1034 In this case, the 0th event is the first date in the list of dates
1035 specified by the rtime. As such, $n must be a positive integer. If
1036 $n is negative, or outside the range of dates specified, the
1037 returned date will be undef (but this is not an error).
1038 2) A very small number of recurrences have an unknown number of
1040 recurring events associated with each date. This only happens if
1041 one of the values in the rtime is specified as a range including
1042 both a positive and negative index. For example, if the day field
1043 in an rtime refers to the day of month, and is 15--15 (i.e. the
1044 15th day to the 15th to the last day), this may include 3 events
1045 (on a month with 31 days), 2 event (months with 30 days), 1 event
1046 (months with 29 days), or 0 events (months with 28 days). As such,
1047 in order to calculate the Nth date, you have to start with the 0th
1048 (i.e. base) date and calculate every event until you get the Nth
1049 one. For this reason, it is highly recommended that this type of
1050 frequency be avoided as it will be quite slow.
1051 3) Most recurrences have a known number of events (equal to the
1053 number of combinations of values in the rtime) for each date. For
1054 these, calculating the Nth date is much faster. However, in this
1055 case, some of them may refer to an invalid date. For example, if
1056 the frequency is 'the 31st of every month' and the base (0th) date
1057 is Jan 31, the 1st event would refer to Feb 31. Since that isn't
1058 valid, undef would be returned for $n=1. Obviously, it would be
1059 possible to actually determine the Nth valid event by calculating
1060 all N-1 dates, but in the interest of performance, this is not
1061 done.
1062 4) The way the Nth recurring event is calculated differs slightly
1064 for NE>0 and N<0 if the delta referred to by the frequency is
1065 approximate. To calculate the Nth recurring event (where N>0), you
1066 take the base date and add N*DELTA (where DELTA is the delta
1067 determined by the frequency). To get the Nth recurring event
1068 (where N<0), a date is determine which, if N*DELTA were added to
1069 it, would produce the base date. For more details, refer to the
1070 Date::Manip::Calc document. In the SUBTRACTION section in the
1071 discussion of approximate date-delta calculations, calculations are
1072 done with $subtract = 2.
1073 next
1075 prev
1076 ($date,$err) = $recur->next();
1077 ($date,$err) = $recur->prev();
1078 These return the next/previous recurring event.
1080 The first time next/prev is called, one of the recurring events
1082 will be selected and returned (using the rules discussed below).
1083 Subsequent calls to next/prev will return the next or previous
1084 event.
1085 Unlike the nth method which will return a specific event (or undef
1087 if the Nth even is not defined), the next and prev methods will
1088 only work with defined events.
1089 So, for the recurrence:
1091 the 31st of every month
1093 next might return the following sequence of events:
1095 Jan 31 2000
1097 Mar 31 2000
1098 May 31 2000
1099 The rules for determining what event to return the first time one
1101 of these is called are as follows:
1102 1) If there is a range, next will return the first event that
1104 occurs after the start of the range. prev will return the last
1105 event that occurs before the end of the range.
1106 2) If there is no range, next will return the first event on or
1108 after the base date. prev will return the last event before the
1109 base date.
1110 The error codes are the same as for the nth method.
1112
1114 I realize that the frequency notation described above looks quite
1115 complicated at first glance, but it is (IMO) the best notation for
1116 expressing recurring events in existence. I actually consider it the
1117 single most important contribution to date/time handling in
1118 Date::Manip.
1119 When I first decided to add recurring events to Date::Manip, I first
1121 came up with a list of common ways of specifying recurring events, and
1122 then went looking for a notation that could be used to define them. I
1123 was hoping for a notation that would be similar to cron notation, but
1124 more powerful.
1125 After looking in several specifications (including ISO 8601) and after
1127 a discussion on a mailing list of calendar related topics, it appeared
1128 that there was no concise, flexible notation for handling recurring
1129 events that would handle all of the common forms I'd come up with.
1130 So, as a matter of necessity, I set about inventing my own notation.
1132 As I was looking at my list, it struck me that all of the parts which
1133 specified a frequency were higher level (i.e. referred to a larger unit
1134 of time) than those parts which specified a specific value (what I've
1135 called the rtime). In other words, when the terms were laid out from
1136 year down to seconds, the frequency part was always left of specific
1137 values.
1138 That led immediately to the notation described above, so I started
1140 analyzing it to figure out if it could express all of the recurring
1141 events I'd come up with. It succeeded on 100% of them. Not only that,
1142 but by playing with different values (especially different combinations
1143 of m/w/d values), I found that it would define recurring events that I
1144 hadn't even thought of, but which seemed perfectly reasonable in
1145 hindsight.
1146 After a very short period, I realized just how powerful this notation
1148 was, and set about implementing it, and as I said above, of all the
1149 contributions that Date::Manip has made, I consider this to be the most
1150 important.
1151
1153 None known.
1154
1156 Please refer to the Date::Manip::Problems documentation for information
1157 on submitting bug reports or questions to the author.
1158
1160 Date::Manip - main module documentation
1161
1163 This script is free software; you can redistribute it and/or modify it
1164 under the same terms as Perl itself.
1165
1167 Sullivan Beck (sbeck@cpan.org)
1168perl v5.10.1 2011-12-07 Date::Manip::Recur(3)