# Copyright (c) 1998-2009 Sullivan Beck. All rights reserved.
# This program is free software; you can redistribute it and/or modify it
# under the same terms as Perl itself.

=pod

=head1 NAME

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

=head1 SYNOPSIS

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

=head1 DESCRIPTION

This module contains functions useful in parsing and manipulating
recurrences.  A recurrence is a notation for defining 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:

=over 4

=item Frequency

The most basic piece of information is a frequency which is the
description of when the event occurs.

Examples 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: 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 repeatedly to get all dates on which a recurring event occurs.
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.

=item Range

In order to actually get a list of dates on which a recurring event
occur, a start and end date are required for all but the most trivial
recurrences.

For example, if the frequency was

   the first of every month

and the start/end dates were Jan 1 2000 and May 31 2000, you could get
the dates:

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

=item Base date

With most frequencies, it is necessary to have a base date (a date on
which the recurring event occurred) in order to determine other dates
when it will occur.

A frequency like:

   the first of every month

does not require a base date, but a frequency like:

   every other Friday

does. Without a base date, it's impossible to determine whether any
given Friday is one in which the event occurs, or one in which it does
not occur.

NOTE: For performance reasons, it is useful (but not required) for the
base date to occur as close to the start of the range as
possible. Placing the base date as the last date on which the event
occurs on or before the start date is ideal. The further the base date
is away from this date, the more intermediate calculations will need
to be done.

=item Modifier

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

For example, in America, many places treat Thanksgiving and the day
after as holidays.

Thanksgiving is easy to define using the frequency:

   4th Thursday of every November

but the day after is NOT possible to define only as a frequency.
Depending on the year, the day after the 4th Thursday may be the 4th
or 5th Friday.

The day after Thanksgiving can be defined as the frequency and modifier:

   4th Thursday of every November
   +1 day

Modifiers can also be used to create events that happen only on
business days.

=back

With these pieces of information, the list of dates in the range can
be obtained where the recurring event occurs.

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

=head1 FREQUENCY NOTATION

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:

  1:2:3:4:5:6:7
  1:2*3:4:5:6:7
 *1:2:3:4:5:6:7

But the following are NOT valid because they contain 2 or more
asterisks:

  1:2*3:4:5*6:7
 *1:2:3:4:5:6*7

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

  1:2*

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 actual events occur.

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

  0:0:0:1:12:0:0

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 rtime:

  *12:0:0

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 most 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:

  *12-13:0,30:0

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
  *1990-1995:12:0:1:0:0:0
                       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--2 in the day element means to go from the 2nd day to the 2nd
to the last day. Currently, negative values are only defined in the
week and day fields.

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 must be 1-7 (it cannot
be a negative number), a range, or a comma separated list.

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 month).

  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

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.

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:

   0:0:0*x:x:x:x
   0:0:1*x:x:x:x

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

=head1 BASE DATES

As mentioned above, base dates are not required for some types of
recurrences.

Any time a frequency refers to every single possible value as specified
by the rtime, no base date is required.

For example, the frequency:

   0:0:1*1:12:0:0      every Monday at noon

refers to every single "Monday at noon" (which is the value specified
by the rtime).

The frequency:

   0:0:2*1:12:0:0      every other Monday at noon

does not, so a base date is required.

The general rule is that if an interval consists of zeros followed by a single
one (i.e. 0:0:1), no base date is required.

A recurrence of the form *Y:M:W:D:H:MN:S also does not use a base date.

The 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.

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.

If a base date is specified for a recurrence which does not require it,
it will be completely ignored.

A default base date is not supplied when a recurrence is created.

=head1 DATE RANGE

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.

=head1 OTHER FREQUENCY FORMATS

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 year.

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 above.

=head1 MODIFIERS

The following modifiers can be used (all of which are case insensitive).

  PDn   : n is 1-7.  Means the previous day n not counting
                     today
  PTn   : n is 1-7.  Means the previous day n counting today
  NDn   : n is 1-7.  Means the next day n not counting today
  NTn   : n is 1-7.  Means the next day n counting today

  FDn   : n is any number.  Means step forward n days.
  BDn   : n is any number.  Means step backward n days.
  FWn   : n is any number.  Means step forward n workdays.
  BWn   : n is any number.  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).

          The CWD. CWM. amd CWP will always change the
          date to the closest work day NOT counting
          today.

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

          The NWD, PWD, and DWD flags all leave the date
          unchanged if it is a work day.

  EASTER: select easter for this year.

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 ignored 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.

There is a practical limitation on how the list of dates are
calculated.

When calculating a list of dates, the first thing is to use the
interval to get a list if dates. The rtime and modifiers are then
applied to this list, and the final list is compared to the start and
end dates, and those dates within this range are returned.

Because dates near the edge of the range may (based on the rtime and
modifiers) move outside the range, and dates just outside the range
may move into the range, dates outside the range have to be added to
the initial list. As a result, when modifiers are present, the first
thing to do is to expand the initial range to include all dates which
will fall inside the actual range (as specified by the begin/end
date).

So, for example, if you have begin and end dates of Jan 10 and Jan 15,
and a modifier of FD2 (forward 2 days), then the interval will be
applied to dates in the range Jan 7 to Jan 14. This is obtained by
applying the FD2 modifier to get Jan 8 to Jan 13 and then adding a 1
day fudge factor on each side to account for any changes due to the
mtime.

The only problem is when applying business day modifiers. Moving
forward 1 business day (with no holidays defined, and only using the
standard weekend definition) may mean moving forward anywhere from 1
to 3 days.  With holidays included, it could theoretically mean moving
forward up to a year (i.e. if there were only 1 work day in the year,
and all others were holidays).

In real life, it won't ever get quite that bad, but it is not at all
unheard of for companies to close for one or two weeks at a time. As
a result, there is no way to know exactly how many days to adjust the
range by to be guaranteed of getting all valid dates.

A best guess is obtained by taking into account the length of the week
and then applying a somewhat arbitrary fudge factor.

For example, if the standard work week is 5 days on, 2 days off, and
you move forward 2 business days, that could be anywhere from 2 to 5
actual days. Finally a fudge factor is applied to make sure that the
range includes all possible days.

The default fudge factor is 5 days, so forward 2 business days would be
treated as forward 2 to 10 actual days, which should be enough to get
every possible date in real life.

The fudge factor can be set to something other than 5 using the
RecurNumFudgeDays config variable.  It it is set to be the total
number of holidays in the year plus 1, it should always yield correct
results, but at some expense.

=head1 DETERMINING DATES

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

=over 4

=item A list of dates is calculated

Based on the interval, the base date, and the range, a list of dates is
calculated from the interval.

The list of dates initially includes all dates that fall inside the range
plus at least one before the range, and at least one after the range.
This allows dates near the edge of the range which might be pushed
across the edge when the rtime values are applied, or modifiers applied.

NOTE: if the recurrence contains no interval (i.e. is of the form
*Y:M:W:D:H:MN:S), no date list is determined. The dates come directly
from the rtime values.

=item The rtime values are applied

All rtime values are applied to the list. Any combination of rtime values
which produce an invalid date are ignored.

For example, if the rtime values refer to the '31st of each month', only
any dates from the list which contain months with 31 days will be used.
The others will be discarded.

=item Modifiers applied

Next, all modifiers are applied.

=item The range is tested

Finally, any dates that fall before or after the range are discarded.

The resulting list of dates is returned.

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
will be returned. The range MAY be specified to return only a subset
of the dates if desired.

=back

=head1 HISTORY OF THE FREQUENCY NOTATION

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
Date::Manip.

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.

After looking in several specifications (including ISO 8601) and after
a discussion on a mailing list of calendar related topics, it appeared
like 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 values.

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 hindsight.

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 important.

=head1 LIST OF Y/M/W/D FREQUENCY DEFINITIONS

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
interval:

  0:0*3:0

is equivalent to

  0:1*3:0

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

=over 4

=item 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.

=item 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

The base date is necessary for all frequencies (except 0:0:1*D) to get
the starting week.

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 possible:

  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

The base date is necessary for all of these (except 0:1*W:D) to get the
starting 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

The base date is necessary only for Y*M:W:D where Y>1.

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
                year

The base date is only required for Y*0:W:D when Y>1 to get the starting
year.

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)

=item 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

The base date is required for all except 0:1*0:D, 1*M:0:D, and *Y:M:0:D
and is used to get the year and month.

=item 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
                FirstDay

  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

=item 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

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

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

  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

=item 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
  1*0:0:0

 *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

=back

=head1 METHODS

=over 4

=item new

=item new_config

=item new_date

=item new_delta

=item new_recur

=item base

=item tz

=item is_date

=item is_delta

=item is_recur

=item config

=item err

Please refer to the Date::Manip::Obj documentation for these methods.

=item parse

   $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:

   FREQ*MODIFIERS*BASE*START*END

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:

   FREQ*MODIFIERS
   FREQ**BASE
   FREQ**BASE*START*END

=item frequency

=item start

=item end

=item base

=item modifiers

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 = $recur->base($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 date.

=item dates

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

Returns the list of dates defined by the full recurrence. If there is
an error, @dates will be empty, and the error flag will be set in the
$recur object.

$start and $end are either undef, or dates which can be used to limit
the set of dates passed back.

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 further limit the dates
returned. In other words, 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.

=back

=head1 KNOWN BUGS

None known.

=head1 BUGS AND QUESTIONS

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

=head1 SEE ALSO

Date::Manip        - main module documentation

=head1 LICENSE

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

=head1 AUTHOR

Sullivan Beck (sbeck@cpan.org)

=cut
