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Date::Manip::Recur(3pmUser Contributed Perl DocumentatiDate::Manip::Recur(3pm)

       Date::Manip::Recur - methods for working with recurring events

          use Date::Manip::Recur;
          $date = new Date::Manip::Recur;

       This module contains functions useful in parsing and manipulating
       recurrences.  A recurrence is a notation for specifying when a
       recurring event occurs.  For example, if an event occurs every other
       Friday or every 4 hours, this can be defined as a recurrence. A fully
       specified recurrence consists of the following pieces of information:

           The most basic piece of information is the frequency.  For
           relatively simple recurring events, the frequency defines when
           those events occur. For more complicated recurring events, the
           frequency tells approximately when the events occur (but to get the
           actual events, certain modifiers must be applied as described

           Examples of recurring events include:

              the first of every month
              every other day
              the 4th Thursday of each month at 2:00 PM
              every 2 hours and 30 minutes

           All of these can be expressed as a frequency.

           NOTE: unlike date parsing, support for frequencies written out in
           English (or whatever language you are working in) is extremely
           limited. For example, the string "the first of every month" will
           NOT be parsed as a valid frequency. A limited number of frequencies
           can be expressed in a written out form (see OTHER FREQUENCY FORMATS
           below), but most must be expressed in the format described below in
           FREQUENCY NOTATION. In this document however, the written out form
           will often be used for the sake of clarity.

           Since a frequency typically refers to events that could happen an
           infinite number of times, you usually have to specify a date range
           to get the actual dates.  Some frequencies also require a base date
           (i.e.  information about when one such even actually occurred)
           since the frequency is ambiguous.  For example, the frequency
           'every other day' does not include enough information to specify
           the dates that the event happened on, so you have to explicitly
           define one.

           Complex recurring events may require the use of modifiers in order
           to get them correct.

           For example, in America, many places treat both Thanksgiving and
           the day after as holidays. Thanksgiving is easy to define since it
           is defined as:

              4th Thursday of every November

           In the frequency notation (described below), this would be written


           The day after Thanksgiving is NOT possible to define in the same
           way.  Depending on the year, the day after the 4th Thursday may be
           the 4th or 5th Friday.

           The only way to accurately define the day after Thanksgiving is to
           specify a frequency and a modifier:

              4th Thursday of every November
              +1 day

           In frequency notation, this can be expressed as:


           The syntax for the various modifiers is described below in the
           MODIFIERS section.

       Base date
           Many recurrences have a base date which is a date on which a
           recurring event is based.

           The base date is not necessarily a date where the recurring event
           occurs. Instead, it may be modified (with modifiers, or with values
           specified in the recurrence) to actually produce a recurring event.

           For example, if the frequency is

              every other Friday at noon

           the base date will be a Friday and the recurring event will happen
           on that Friday, Friday two weeks later, Friday four weeks later,
           etc.  In all cases, the dates will be modified to be at noon.

           If the frequency has a modifier, such as:

              every other Friday
              + 1 day

           (and yes, this trivial example could be expressed as the frequency
           'every other Saturday' with no modifiers), then the base date is
           still on a Friday, but the actual recurring event is determined by
           applying modifiers and occurs on Saturday.

           Recurring events are assigned a number with the event that is
           referred to by the base date being the 0th occurrence, the first
           one after that as the 1st occurrence, etc.  Recurring events can
           also occur before the base date with the last time the recurring
           event occurred before the base date is the -1th occurence.

           So, if the frequency is

              the first of every month

           and the base date is 'Mar 1, 2000', then the 5 recurring events
           around it are:

              N    Date

              -2   Jan 1 2000
              -1   Feb 1 2000
               0   Mar 1 2000
              +1   Apr 1 2000
              +2   May 1 2000

           In some cases, the Nth date may not be defined. For example, if the
           frequency is:

              the 31st of every month

           and the base date is Mar 31, 2000, the 5 recurring events around it

              N   Date

              -2  Jan 31 2000
              -1  undefined
               0  Mar 31 2000
               1  undefined
               2  May 31 2000

           As mentioned above, the base date is used to determine one of the
           occurrences of the recurring event... but it may not actually be on
           of those events.

           As an example, for the recurring event:

              every other Friday

           a base date could be on a Friday, but it would also be possible to
           have a base date on some other day of the week, and it could
           unambiguously refer simply to a week, and the recurring event would
           occur on Friday of that week.

           In most cases, it won't be necessary to treat base dates with that
           level of complexity, but with complicated recurring events, it may
           be necessary.  More information on how Date::Manip determines a
           recurring event from a base date is given below in the section BASE

           A date range is simply a starting and an ending date. When a range
           is used (primarily in the dates method as described below), only
           recurring events (with all modifiers applied) which happened on or
           after the start date and on or before the end date are used.

           For example, if the frequency was

              the first of every month

           and the start/end dates were Jan 1 2000 and May 31 2000, the list
           of dates referred to would be:

              Jan 1 2000
              Feb 1 2000
              Mar 1 2000
              Apr 1 2000
              May 1 2000

           If no base date is specified, but a date range is specified, the
           start date is used as the specified base date.

           It should be noted that if both the range and base date are
           specified, the range is not used to determine a base date. Also,
           the first time the recurring event occurs in this range may NOT be
           the 0th occurrence with respect to the base date, and that is

           NOTE: both dates in the range and the base date must all be in the
           same time zone, and use the same Date::Manip::Base object.

       The syntax for specifying a frequency requires some explanation. It is
       very concise, but contains the flexibility to express every single type
       of recurring event I could think of.

       The syntax of the frequency description is a colon separated list of
       the format Y:M:W:D:H:MN:S (which stand for year, month, week, etc.).
       One (and only one) of the colons may optionally be replaced by an
       asterisk, or an asterisk may be prepended to the string.  For example,
       the following are all valid frequency descriptions:


       But the following are NOT valid because they contain more than one


       When an asterisk is included, the portion to the left of it is called
       the interval, and refers to an approximate time interval between
       recurring events.  For example, if the interval of the frequency is:


       it means that the recurring event occurs approximately every 1 year and
       2 months.  The interval is approximate because elements to the right of
       the asterisk, as well as any modifiers included in the recurrence, will
       affect when the events actually occur.

       If no asterisks are included, then the entire recurrence is an
       interval.  For example,


       refers to an event that occurs every 1 day, 12 hours.

       The portion of the frequency that occur after an asterisk is called the
       recurrence time (or rtime), and refers to a specific value (or values)
       for that type of time element (i.e. exactly as it would appear on a
       calendar or a clock).  For example, if the frequency ends with the


       then the recurring event occurs at 12:00:00 (noon).

       For example:

         0:0:0:2*12:30:0      every 2 days at 12:30 (each day)

       Elements in the rtime can be listed as single values, ranges (2 numbers
       separated by a dash "-"), or a comma separated list of values or
       ranges.  In some cases, negative values are appropriate for the week or
       day values. -1 stands for the last possible value, -2 for the second to
       the last, etc.

       If multiple values are included in more than one field in the rtime,
       every possible combination will be used. For example, if the frequency
       ends with the rtime:


       the event will occur at 12:00, 12:30, 13:00, and 13:30.

       Some examples are:

         0:0:0:1*2,4,6:0:0    every day at at 02:00, 04:00, and 06:00
         0:0:0:2*12-13:0,30:0 every other day at 12:00, 12:30, 13:00,
                              and 13:30
         0:1:0*-1:0:0:0       the last day of every month
                              Dec 1 in 1990 through 1995

       There is no way to express the following with a single recurrence:

         every day at 12:30 and 1:00

       You have to use two recurrences to do this.

       You can include negative numbers in ranges. For example, including the
       range -2---1 means to go from the 2nd to the last to the last
       occurrence.  Negative values are only supported in the week and day
       fields, and only in some cases.

       You can even use a range like 2--2 (which means to go from the 2nd to
       the 2nd to the last occurrence). However, this is STRONGLY discouraged
       since this leads to a date which produces a variable number of events.
       As a result, the only way to determine the Nth date is to calculate
       every date starting at the base date. If you know that every date
       produces exactly 4 recurring events, you can calculate the Nth date
       without needing to determine every intermediate date.

       When specifying a range, the first value must be less than the second
       or else nothing will be returned.

       When both the week and day elements are non-zero and the day is right
       of the asterisk, the day refers to the day of week. The following
       examples illustrate these type of frequencies:

         0:1*4:2:0:0:0        4th Tuesday (day 2) of every month
         0:1*-1:2:0:0:0       last Tuesday of every month
         0:0:3*2:0:0:0        every 3rd Tuesday (every 3 weeks
                              on 2nd day of week)
         1:0*12:2:0:0:0       the 12th Tuesday of each year

       NOTE: The day of week refers to the numeric value of each day as
       specified by ISO 8601. In other words, day 1 is ALWAY Monday, day 7 is
       ALWAYS Sunday, etc., regardless of what day of the week the week is
       defined to begin on (using the FirstDay config variable). So when the
       day field refers to the day of week, it's value (or values if a range
       or comma separated list are used) must be 1-7.

       When the week element is zero and the month element is non-zero and the
       day element is right of the asterisk, the day value is the day of the
       month (it can be from 1 to 31 or -1 to -31 counting from the end of the

         3*1:0:2:12:0:0       every 3 years on Jan 2 at noon
         0:1*0:2:12,14:0:0    2nd of every month at 12:00 and 14:00
         0:1:0*-2:0:0:0       2nd to last day of every month

       NOTE: If the day given refers to the 29th, 30th, or 31st, in a month
       that does not have that number of days, it is ignored. For example, if
       you ask for the 31st of every month, it will return dates in Jan, Mar,
       May, Jul, etc.  Months with fewer than 31 days will be ignored.

       If both the month and week elements are zero, and the year element is
       non-zero, the day value is the day of the year (1 to 365 or 366 -- or
       the negative numbers to count backwards from the end of the year).

         1:0:0*45:0:0:0       45th day of every year

       Specifying a day that doesn't occur in that year silently ignores that
       year. The only result of this is that specifying +366 or -366 will
       ignore all years except leap years.

       If the week element is non-zero and to the right of the asterisk, and
       the day element is zero, the frequency refers to the first day of the
       given week of the month or week of the year:

         0:1*2:0:0:0:0        the first day of the 2nd week of
                              every month
         1:0*2:0:0:0:0        the first day of the 2nd week of
                              every year

       Although the meaning of almost every recurrence can be deduced by the
       above rules, a set of tables describing every possible combination of
       Y/M/W/D meanings, and giving an example of each is included below in
       the section LIST OF Y/M/W/D FREQUENCY DEFINITIONS. It also explains a
       small number of special cases.

       NOTE: If all fields left of the asterisk are zero, the last one is
       implied to be 1. In other words, the following are equivalent:


       and can be thought of as every possible occurence of the rtime.

       NOTE: When applying a frequency to get a list of dates on which a
       recurring event occurs, a delta is created from the frequency which is
       applied to get dates referred to by the interval. These are then
       operated on by the rtime and by modifiers to actually get the recurring
       events.  The deltas will always be exact or approximate.  There is no
       support for business mode recurrences. However, with the careful use of
       modifiers (discussed below), most recurring business events can be
       determined too.

       A recurrence of the form *Y:M:W:D:H:MN:S (which is technically speaking
       not a recurring event... it is just a date or dates specified using the
       recurrence syntax) uses the first date which matches the frequency as
       the base date. Any base date specified will be completely ignored. A
       date range may be specified to work with a subset of the dates.

       All other recurrences use a specified base date in order to determine
       when the 0th occurrence of a recurring event happens. As mentioned
       above, the specified base date may be determined from the start date,
       or specified explicitly.

       The specified base date is used to provide the bare minimum
       information. For example, the recurrence:

          0:0:3*4:0:0:0       every 3 weeks on Thursday

       requires a base date to determine the week, but nothing else. Using the
       standard definition (Monday-Sunday) for a week, and given that one week
       in August 2009 is Aug 10 to Aug 16, any date in the range Aug 10 to Aug
       16 will give the same results. The definition of the week defaults to
       Monday-Sunday, but may be modified using the FirstDay config variable.

       Likewise, the recurrence:

         1:3*0:4:0:0:0        every 1 year, 3 months on the 4th
                              day of the month

       would only use the year and month of the base date, so all dates in a
       given month would give the same set of recurring dates.

       It should also be noted that a date may actually produce multiple
       recurring events. For example, the recurrence:

          0:0:2*4:12,14:0:0   every 2 weeks on Thursday at 12:00
                              and 14:00

       produces 2 events for every date. So in this case, the base date
       produces the 0th and 1st event, the base date + an offset produces the
       2nd and 3rd events, etc.

       It must be noted that the base date refers ONLY to the interval part of
       the recurrence. The rtime and modifiers are NOT used in determining the
       base date.

       The interval of a frequency (everything left of the asterisk) will be
       used to generate a list of dates (called interval dates). When rtime
       values and modifiers are applied to an interval date, it produces the
       actual recurring events.

       As already noted, if the rtime values include multiple values for any
       field, more than one event are produced by a single interval date.

       It is important to understand is how the interval dates are calculated.
       The interval is trivially turned into a delta. For example, with the
       frequency 0:0:2*4:12:0:0, the interval is 0:0:2 which produces the
       delta 0:0:2:0:0:0:0.

       In order to get the Nth interval date, the delta is multiplied by N and
       added to the base date. In other words:

          D(0) = Jan 31
          D(1) = Jan 31 + 1 month = Feb 28
          D(2) = Jan 31 + 2 month = Mar 31

       The start and end dates form the range in which recurring events can
       fall into.

       Every recurring date will fall in the limit:

          start <= date <= end

       When a recurrence is created, it may include a default range, and this
       is handled by the RecurRange config variable.

       There are a small handful of English strings (or the equivalent in
       other languages) which can be parsed in place of a numerical frequency.
       These include:

         every Tuesday in June [1997]
         2nd Tuesday in June [1997]
         last Tuesday in June [1997]

         every Tuesday of every month [in 1997]
         2nd Tuesday of every month [in 1997]
         last Tuesday of every month [in 1997]

         every day of every month [in 1997]
         2nd day of every month [in 1997]
         last day of every month [in 1997]

         every day [in 1997]
         every 2nd day [in 1977]
         every 2 days [in 1977]

       Each of these set the frequency. If the year is include in the string,
       it also sets the dates in the range to be the first and last day of the

       In each of these, the numerical part (i.e. 2nd in all of the examples
       above) can be any number from 1 to 31. To make a frequency with a
       larger number than that, you have to use the standard format discussed

       Due to the complexity of writing out (and parsing) frequencies written
       out, I do not intend to add additional frequency formats, and the use
       of these is discouraged. The frequency format described above is

       Any number of modifiers may be added to a frequency to get the actual
       date of a recurring event.  Modifiers are case sensitive.

       Modifiers to set the day-of-week
           The following modifiers can be used to adjust a date to a specific
           day of the week.

             PDn   Means the previous day n not counting today
             PTn   Means the previous day n counting today
             NDn   Means the next day n not counting today
             NTn   Means the next day n counting today
             WDn   Day n (1-7) of the current week

           In each of these, 'n' is 1-7 (1 being Sunday, 7 being Saturday).

           For example, PD2/ND2 returns the previous/next Tuesday. If the date
           that this is applied to is Tuesday, it modifies it to one week in
           the past/future.

           PT2/NT2 are similar, but will leave the date unmodified if it is a

       Modifiers to move forward/backward a number of days
           These modifiers can be used to add/subtract n days to a date.

             FDn   Means step forward n days.
             BDn   Means step backward n days.

       Modifiers to force events to be on business days
           Modifiers can also be used to force recurring events to occur on
           business days. These modifiers include:

             FWn   Means step forward n workdays.
             BWn   Means step backward n workdays.

             CWD   The closest work day (using the TomorrowFirst
                   config variable).
             CWN   The closest work day (looking forward first).
             CWP   The closest work day (looking backward first).

             NWD   The next work day counting today
             PWD   The previous work day counting today
             DWD   The closest work day (using the TomorrowFirst config
                   variable) counting today

             IBD   This discards the date if it is not a business day.
             NBD   This discards the date if it IS a business day.

           The CWD, CWN, and CWP modifiers will always change the date to the
           closest working day NOT counting the current date.

           The NWD, PWD, and DWD modifiers always change the date to the
           closest working day unless the current date is a work day. In that
           case, it is left unmodified.

           CWD, CWN, and CWP will usually return the same value, but if you
           are starting at the middle day of a 3-day weekend (for example), it
           will return either the first work day of the following week, or the
           last work day of the previous week depending on whether it looks
           forward or backward first.

           All business day modifiers ignore the time, so if a date is
           initially calculated at Saturday at noon, and the FW1 is applied,
           the date is initially moved to the following Monday (assuming it is
           a work day) and the FW1 moves it to Tuesday. The final result will
           be Tuesday at noon.

           The IBD and NBD modifiers eliminate dates from the list
           immediately.  In other words, if a recurrence has three modifiers:


           then as a date is being tested, first the FD1 modifier is applied.
           Then, it is tested to see if it is a business day.  If it is, the
           second FD1 modifier will be applied.  Otherwise, the date will not
           be included in the list of recurring events.

       Special modifiers
           The following modifiers do things that cannot be expressed using
           any other combination of frequency and modifiers:

             EASTER   Set the date to Easter for this year.

       In order to get a list of dates referred to by the recurrence, the
       following steps are taken.

       The recurrence is tested for errors
           The recurrence must be completely specified with a base date
           (either supplied explicitly, or derived from a start date) and date
           range when necessary. All dates must be valid.

       The actual base date is determined
           Using information from the interval and the specified base date,
           the actual base date is determined.

       The Nth date is calculated
           By applying the delta that corresponds to the interval, and then
           applying rtime and modifier information, the Nth date is

           This is repeated until all desired dates have been obtained.

           The nth method described below has more details.

       The range is tested
           Any date that fall outside the range is discarded.

           NOTE: when the recurrence contains no interval, it is not necessary
           to specify the range, and if it is not specified, all of the dates
           are used. The range MAY be specified to return only a subset of the
           dates if desired.

       Because the week and day values may have multiple meanings depending on
       where the asterisk is, and which of the fields have non-zero values, a
       list of every possible combination is included here (though most can be
       determined using the rules above).

       When the asterisk occurs before the day element, and the day element is
       non-zero, the day element can take on multiple meanings depending on
       where the asterisk occurs, and which leading elements (year, month,
       week) have non-zero values. It can refer to the day of the week, day of
       the month, or day of the year.

       When the asterisk occurs before the week element, the week element of
       the frequency can also take on multiple meanings as well. When the
       month field and day fields are zero, it refers to the week of the year.
       Since the week of the year is well defined in the ISO 8601 spec, there
       is no ambiguity.

       When the month field is zero, but the day field is not, the week field
       refers to the nth occurrence of the day of week referred to by the day
       field in the year.

       When the month field is non-zero, the week field refers to the nth
       occurrence of the day of week in the month.

       In the tables below only the first 4 elements of the frequency are
       shown. The actual frequency will include the hour, minute, and second
       elements in addition to the ones shown.

       When all elements left of the asterisk are 0, the interval is such that
       it occurs the maximum times possible (without changing the type of
       elements to the right of the asterisk). Another way of looking at it is
       that the last 0 element of the interval is changed to 1. So, the


       is equivalent to


       When the year field is zero, and is right of the asterisk, it means the
       current year.

       All elements left of the asterisk
           When all of the month, week, and day elements are left of the
           asterisk, the simple definitions of the frequency are used:

             frequency     meaning

             1:2:3:4       every 1 year, 2 months, 3 weeks,
                           4 days

           Any, or all of the fields can be zero.

       Non-zero day, non-zero week
           When both the day and week elements are non-zero, the day element
           always refers to the day of week. Values must be in the range (1 to
           7) and no negative values are allowed.

           The following tables shows all possible variations of the frequency
           where this can happen (where day 4 = Thursday).

           When the week is left of the asterisk, the interval is used to get
           the weeks on the calendar containing a recurring date, and the day
           is used to set the day of the week.  The following are possible:

             frequency     meaning

             1:2:3*4       every 1 year, 2 months, 3 weeks
                           on Thur

             1:0:3*4       every 1 year, 3 weeks on Thur

             0:2:3*4       every 2 months, 3 weeks on Thur

             0:0:3*4       every 3 weeks on Thur

           When the week is right of the asterisk, and a non-zero month is
           left of the asterisk, the recurrence refers to a specific
           occurrence of a day-of-week during a month. The following are

             frequency     meaning

             1:2*3:4       every 1 year, 2 months on the
                           3rd Thursday of the month

             0:2*3:4       every 2 months on the 3rd Thur
                           of the month

           When the week and month are both non-zero and right of the
           asterisk, the recurrence refers to an occurrence of day-of-week
           during the given month.  Possibilities are:

             frequency     meaning

             1*2:3:4       every 1 year in February on
                           the 3rd Thur

             0*2:3:4       same as 1*2:3:4

            *1:2:3:4       in Feb 0001 on the 3rd Thur
                           of the month

            *0:2:3:4       on the 3rd Thur of Feb in the
                           current year

           When the week is right of the asterisk, and the month is zero, the
           recurrence refers to an occurence of the day-of-week during the
           year. The following are possible:

             frequency     meaning

             1:0*3:4       every 1 year on the 3rd Thursday
             1*0:3:4       of the year

            *1:0:3:4       in 0001 on the 3rd Thur of
                           the year

             0*0:3:4       same as 1*0:3:4

            *0:0:3:4       on the 3rd Thur of the current

           There is one special case:

             frequency     meaning

             0:0*3:4       same as 0:1*3:4 (every month on
                           the 3rd Thur of the month)

       Non-zero day, non-zero month
           When a non-zero day element occurs to the right of the asterisk and
           the week element is zero, but the month element is non-zero, the
           day elements always refers to a the day of month in the range (1 to
           31) or (-1 to -31).

           The following table shows all possible variations of the frequency
           where this can happen:

             frequency     meaning

             1:2:0*4       every 1 year, 2 months on the
             1:2*0:4       4th day of the month

             1*2:0:4       every year on Feb 4th

            *1:2:0:4       Feb 4th, 0001

             0:2:0*4       every 2 months on the 4th day
             0:2*0:4       of the month

             0*2:0:4       same as 1*2:0:4

            *0:2:0:4       Feb 4th of the current year

       Zero day, non-zero week
           When a day is zero, and the week is non-zero, the recurrence refers
           to a specific occurrence of the first day of the week (as given by
           the FirstDay variable).

           The frequency can refer to an occurrence of FirstDay in a specific
           week (if the week is left of the asterisk):

             frequency     meaning

             1:2:3*0       every 1 year, 2 months, 3 weeks on

             1:0:3*0       every 1 year, 3 weeks on FirstDay

             0:2:3*0       every 2 months, 3 weeks on FirstDay

             0:0:3*0       every 3 weeks on FirstDay

           or to a week in the year (if the week is right of the asterisk, and
           the month is zero):

             frequency     meaning

             1:0*3:0       every 1 year on the first day of the
             1*0:3:0       3rd week of the year

            *1:0:3:0       the first day of the 3rd week of 0001

           or to an occurrence of FirstDay in a month (if the week is right of
           the asterisk and month is non-zero):

             frequency     meaning

             1:2*3:0       every 1 year, 2 months on the 3rd
                           occurence of FirstDay

             0:2*3:0       every 2 months on the 3rd occurence
                           of FirstDay

             1*2:3:0       every year on the 3rd occurence
                           of FirstDay in Feb

             0*2:3:0       same as 1*2:3:0

            *1:2:3:0       the 3rd occurence of FirstDay
                           Feb 0001

            *0:2:3:0       the 3rd occurence of FirstDay
                           in Feb of the current year

           NOTE: in the last group, a slightly more intuitive definition of
           these would have been to say that the week field refers to the week
           of the month, but given the ISO 8601 manner of defining when weeks
           start, this definition would have virtually no practical
           application. So the definition of the week field referring to the
           Nth occurence of FirstDay in a month was used instead.

           There are a few special cases here:

             frequency     meaning

             0:0*3:0       same as 0:1*3:0   (every month on the 3rd
                           occurence of the first day of week)

             0*0:3:0       same as 1*0:3:0

            *0:0:3:0       the first day of the 3rd week of the
                           current year

       Non-zero day
           When a non-zero day element occurs and both the month and week
           elements are zero, the day elements always refers to a the day of
           year (1 to 366 or -1 to -366 to count from the end).

           The following table shows all possible variations of the frequency
           where this can happen:

             frequency     meaning

             1:0:0*4       every year on the 4th day of
             1:0*0:4       the year

            *1:0:0:4       the 4th day of 0001

           Other non-zero day variations have multiple meanings for the day

             frequency     meaning

             0:0:0*4       same as 0:0:1*4  (every week on Thur)

             0:0*0:4       same as 0:1*0:4  (every month on the 4th)

             0*0:0:4       same as 1*0:0:4

            *0:0:0:4       the 4th day of the current year

       All other variations
           The remaining variations have zero values for both week and day.
           They are:

             frequency     meaning

             1:2:0*0       every 1 year, 2 months on the first
             1:2*0:0       day of the month

             1*2:0:0       every year on Feb 1

            *1:2:0:0       Feb 1, 0001

             1:0:0*0       every 1 year on Jan 1

            *1:0:0:0       Jan 1, 0001

             0:2:0*0       every 2 months on the first day of
             0:2*0:0       the month

             0*2:0:0       same as 1*2:0:0

            *0:2:0:0       Feb 1 of the current year

             0:0:0*0       same as 0:0:1*0 (every week on
                           the first day of the week)

             0:0*0:0       same as 0:1*0:0 (every month
                           on the 1st)

             0*0:0:0       same as 1*0:0:0

            *0:0:0:0       Jan 1 of the current year

       err Please refer to the Date::Manip::Obj documentation for these

              $err = $recur->parse($string [,$modifiers] [,$base,$start,$end]);

           This creates a new recurrence. A string containing a valid
           frequency is required. In addition, $start, $end, and $base dates
           can be passed in (either as Date::Manip::Date objects, or as
           strings containing dates that can be parsed), and any number of the
           modifiers listed above.

           If the $start or $end dates are not included, they may be supplied
           automatically, based on the value of the RecurRange variable. If
           any of the dates are passed in, they must be included in the order
           given (though it is safe to pass an empty string or undef in for
           any of them if you only want to set some, but not all of them).

           The $modifiers argument must either contain valid modifiers, or be
           left out of the argument list entirely. You cannot pass an empty
           string or undef in for it.

              $err = $recur->parse($string);

           This creates a recurrence from a string which contains all of the
           necessary elements of the recurrence. The string is of the format:


           where FREQ is a string containing a frequency, MODIFIERS is a
           string containing a comma separated list of modifiers, BASE, START,
           and END are strings containing parseable dates.

           All pieces are optional, but order must be maintained, so all of
           the following are valid:


           If a part of the recurrence is passed in both as part of $string
           and as an argument, the argument overrides the string portion, with
           the possible exception of modifiers. The modifiers in the argument
           override the string version unless the first one is a '+' in which
           case they are appended. See the modifiers method below for more

           You can also create a recurrency in steps (or replace parts of an
           existing recurrence) using the following:

              $err = $recur->frequency($frequency);

              $err = $recur->start($start);
              $err = $recur->end($end);

              $err = $recur->basedate($base);

              $err = $recur->modifiers($modifiers);
              $err = $recur->modifiers(@modifiers);

           These set the appropriate part of the recurrence.

           Calling the frequency method discards all information currently
           stored in the Recur object (including an existing start, end, and
           base date), so this method should be called first.

           In the modifiers method, the modifiers can be passed in as a string
           containing a comma separated list of modifiers, or as a list of
           modifiers. The modifiers passed in override all previously set
           modifiers UNLESS the first one is the string "+", in which case the
           new modifiers are appended to the list.

           In the start, end, and base methods, the date passed in can be a
           Date::Manip::Date object, or a string that can be parsed to get a

           NOTE: the parse method will overwrite all parts of the recurrence,
           so it is not appropriate to do:


           The modifiers passed in in the first call will be overwritten.

           These functions can also be used to look up the values.

              $freq  = $recur->frequency();
              $start = $recur->start();
              $end   = $recur->end();
              @mods  = $recur->modifiers();

              ($base,$actual) = $recur->basedate();

           The basedate function will return both the specified base and the
           actual base dates.

           If any of the values are not yet determined, nothing will be

              @dates = $recur->dates([$start,$end]);

           Returns the list of dates defined by the full recurrence. If there
           is an error, or if there are no dates, an empty list will be

           $start and $end are either undef, or dates which can be used to
           limit the set of dates passed back (they can be Date::Manip::Date
           objects or strings that can be parsed).

           If the recurrence does not have a start and end date already,
           passing in $start and $end will set the range (but they will NOT be
           stored in the recurrence).

           If the recurrence does have a start and end date stored in it, the
           $start and $end arguments can be used to temporarily override the
           limits. For example, if a recurrence has a start date of Jan 1,
           2006 00:00:00 and and end date of Dec 31, 2006 23:59:59 stored in
           the recurrence, passing in $start of Jul 1, 2006 00:00:00 will
           limit the dates returned to the range of Jul 1 to Dec 31.

           Passing in a start date of Jul 1, 2007 will mean that no dates are
           returned since the recurrence limits the date to be in 2006.

           If one or both of $start and $end are undef, then the stored values
           will be used.

              ($date,$err) = $recur->nth($n);

           This returns the $n'th recurring event ($n may be any integer). If
           an error occurs, it is returned (but it is not set in $recur since
           it may be properly, though perhaps incompletely, defined). The
           following errors may be returned:

              Invalid recurrence
                 The recurrence has an error flag set.

              Incomplete recurrence
                 The recurrence is incomplete. It needs either a
                 base date or a date range.

              Range invalid
                 The recurrence has an invalid date range (i.e.
                 the end date occurs before the start date).

              Start invalid
              End invalid
              Base invalid
                 An invalid date was entered for one of the dates.

           There are a few special circumstances to be aware of.

           1) If the recurrence contains no interval (i.e. is of the form
           *Y:M:W:D:H:MN:S), the dates come directly from the rtime values.
           In this case, the 0th event is the first date in the list of dates
           specified by the rtime. As such, $n must be a positive integer.  If
           $n is negative, or outside the range of dates specified, the
           returned date will be undef (but this is not an error).

           2) A very small number of recurrences have an unknown number of
           recurring events associated with each date.  This only happens if
           one of the values in the rtime is specified as a range including
           both a positive and negative index.  For example, if the day field
           in an rtime refers to the day of month, and is 15--15 (i.e. the
           15th day to the 15th to the last day), this may include 3 events
           (on a month with 31 days), 2 event (months with 30 days), 1 event
           (months with 29 days), or 0 events (months with 28 days). As such,
           in order to calculate the Nth date, you have to start with the 0th
           (i.e. base) date and calculate every event until you get the Nth
           one. For this reason, it is highly recommended that this type of
           frequency be avoided as it will be quite slow.

           3) Most recurrences have a known number of events (equal to the
           number of combinations of values in the rtime) for each date. For
           these, calculating the Nth date is much faster. However, in this
           case, some of them may refer to an invalid date. For example, if
           the frequency is 'the 31st of every month' and the base (0th) date
           is Jan 31, the 1st event would refer to Feb 31. Since that isn't
           valid, undef would be returned for $n=1. Obviously, it would be
           possible to actually determine the Nth valid event by calculating
           all N-1 dates, but in the interest of performance, this is not

           4) The way the Nth recurring event is calculated differs slightly
           for NE>0 and N<0 if the delta referred to by the frequency is
           approximate. To calculate the Nth recurring event (where N>0), you
           take the base date and add N*DELTA (where DELTA is the delta
           determined by the frequency).  To get the Nth recurring event
           (where N<0), a date is determine which, if N*DELTA were added to
           it, would produce the base date. For more details, refer to the
           Date::Manip::Calc document.  In the SUBTRACTION section in the
           discussion of approximate date-delta calculations, calculations are
           done with $subtract = 2.

              ($date,$err) = $recur->next();
              ($date,$err) = $recur->prev();

           These return the next/previous recurring event.

           The first time next/prev is called, one of the recurring events
           will be selected and returned (using the rules discussed below).
           Subsequent calls to next/prev will return the next or previous

           Unlike the nth method which will return a specific event (or undef
           if the Nth even is not defined), the next and prev methods will
           only work with defined events.

           So, for the recurrence:

              the 31st of every month

           next might return the following sequence of events:

              Jan 31 2000
              Mar 31 2000
              May 31 2000

           The rules for determining what event to return the first time one
           of these is called are as follows:

           1) If there is a range, next will return the first event that
           occurs after the start of the range.  prev will return the last
           event that occurs before the end of the range.

           2) If there is no range, next will return the first event on or
           after the base date.  prev will return the last event before the
           base date.

           The error codes are the same as for the nth method.

       I realize that the frequency notation described above looks quite
       complicated at first glance, but it is (IMO) the best notation for
       expressing recurring events in existence. I actually consider it the
       single most important contribution to date/time handling in

       When I first decided to add recurring events to Date::Manip, I first
       came up with a list of common ways of specifying recurring events, and
       then went looking for a notation that could be used to define them.  I
       was hoping for a notation that would be similar to cron notation, but
       more powerful.

       After looking in several specifications (including ISO 8601) and after
       a discussion on a mailing list of calendar related topics, it appeared
       that there was no concise, flexible notation for handling recurring
       events that would handle all of the common forms I'd come up with.

       So, as a matter of necessity, I set about inventing my own notation.
       As I was looking at my list, it struck me that all of the parts which
       specified a frequency were higher level (i.e. referred to a larger unit
       of time) than those parts which specified a specific value (what I've
       called the rtime). In other words, when the terms were laid out from
       year down to seconds, the frequency part was always left of specific

       That led immediately to the notation described above, so I started
       analyzing it to figure out if it could express all of the recurring
       events I'd come up with. It succeeded on 100% of them. Not only that,
       but by playing with different values (especially different combinations
       of m/w/d values), I found that it would define recurring events that I
       hadn't even thought of, but which seemed perfectly reasonable in

       After a very short period, I realized just how powerful this notation
       was, and set about implementing it, and as I said above, of all the
       contributions that Date::Manip has made, I consider this to be the most

       If you specify a recurrence which cannot be satisfied for the base
       date, or for any time after the base date, the recurrence will crash.
       This can only happen if you specify a recurrence that always occurs in
       the spring DST transition using the current timezone rules.

       For example, in a US timezone, the current timezone rules state that a
       DST transition occurs at 02:00:00 on the 2nd Sunday in March and the
       clock jumps to 03:00.  This started in 2006.  As a result, the


       with a base date of 2006 or later cannot be satisfied.

       Please refer to the Date::Manip::Problems documentation for information
       on submitting bug reports or questions to the author.

       Date::Manip        - main module documentation

       This script is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       Sullivan Beck (

perl v5.20.0                      2014-09-14           Date::Manip::Recur(3pm)

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