Data structure for storing recurring events?

Question

I'm looking for a data structure pattern for storing recurring events, but everything I came up with would result in a high number of special case handling or user input and data retrieval are overly complex. (I get the distinct feeling, that I haven't understand the problem domain well enough to do this.)

How can I store Outlook-style recurring events?

• Every day at 8am
• Every first tuesday in a month
• Every December 1st for three years
• Every two hours for a week
• ...

Answer 1

There are various papers describing data structures and algorithms for this use case. In addition you can see the code or descriptions of open source implementation of crontab and of Quartz (Java) or Quartz.NET (.NET).

This is one such paper

http://portal.acm.org/citation.cfm?id=359763.359801&coll=ACM&dl=ACM&CFID=63647367&CFTOKEN=55814330

For example, cron stores the information like this (* means every, so a * under month means every month)

.---------------- minute (0 - 59) 
|  .------------- hour (0 - 23)
|  |  .---------- day of month (1 - 31)
|  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ... 
|  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7)  OR sun,mon,tue,wed,thu,fri,sat 
|  |  |  |  |
*  *  *  *  * 

There are several special entries, most of which are just shortcuts, 
that can be used instead of specifying the full cron entry:

Entry      Description                 Equivalent To
@reboot    Run once, at startup.       None
@yearly    Run once a year             0 0 1 1 *
@annually  (same as @yearly)           0 0 1 1 *
@monthly   Run once a month            0 0 1 * *
@weekly    Run once a week             0 0 * * 0
@daily     Run once a day              0 0 * * *
@midnight  (same as @daily)            0 0 * * *
@hourly    Run once an hour         0 * * * *

Answer 2

Event:

StartDate
EndDate (calculated on change of NumberOfOccurances)
NumberOfOccurances (calculated on change of EndDate )
Frequency e.g. 1/2hrs, 1/month, 1/day, ....
CorrectionFunction e.g. first Tuesday, last Sunday, ...

bool OccuresOn(day)
Date NextOccurance(date)

Answer 3

Support the standard iCalendar Event types

The IETF put some thought into this when they created the Internet Calendaring and Scheduling Core Object Specification, better known as iCalendar.

The specification includes event recurrence.

As an added bonus, your database will be amenable to sharing data with other iCalendar compatible data sources such as Google and Apple calendars.

http://tools.ietf.org/html/rfc5545