- NAME
- SYNOPSIS
- DESCRIPTION
- FREQUENCY NOTATION
- BASE DATES
- DATE RANGE
- OTHER FREQUENCY FORMATS
- MODIFIERS
- DETERMINING DATES
- HISTORY OF THE FREQUENCY NOTATION
- LIST OF Y/M/W/D FREQUENCY DEFINITIONS
- METHODS
- KNOWN BUGS
- BUGS AND QUESTIONS
- SEE ALSO
- LICENSE
- AUTHOR

# NAME

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

# SYNOPSIS

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

# 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:

**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.

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 small 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 the documentation however, the written out form will often be used for the sake of clarity.

**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, the list of dates would include:

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

**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.

**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.

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

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.

# 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 an approximate 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.

# 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.

# 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.

# 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.

# 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.

# DETERMINING DATES

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

**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.

**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.

**Modifiers applied**-
Next, all modifiers are applied.

**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.

# 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 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 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.

# 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.

**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`

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)`

**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.

**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`

**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`

**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`

# METHODS

**new****new_config****new_date****new_delta****new_recur****base****tz****is_date****is_delta****is_recur****config****err**-
Please refer to the Date::Manip::Obj documentation for these methods.

**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`

**frequency****start****end****base****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.

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

`$recur->modifiers($modifiers); $recur->parse($string);`

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

**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.

# KNOWN BUGS

None known.

# BUGS AND QUESTIONS

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

# SEE ALSO

Date::Manip - main module documentation

# LICENSE

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

# AUTHOR

Sullivan Beck (sbeck@cpan.org)