I think I understand the basics of how cron would call a perl script, which would talk to a database ... but I'm trying to think how the database would be set up and work.

For any given event, "December 31st at 9 PM, do X" it's not that hard -- I guess the system would fire at 9, select the event based on its time being less-than-or-equal-to the current time (and its status as not completed), do whatever it was, then mark it as having completed.

But a truly useful system would have something where users could put in things like "every second Friday", "the second Wednesday of every month", "every Tuesday in March" and also need to have spans of time, i.e. "every second Friday, ending on January 31st" -- and I think the system would be even better if it could allow for exceptions, like "every Friday, but NOT Friday the 26th of June".

Is there anything pre-written out there? Some kind of object-mapping? What are the normal design patterns for a thing like this -- populate a database with multiple dates when a recurring event is added? Have a database of exceptions to events which are allowed to over-rule the normal events?

As I said in the title, it's not exactly a perl question but I know the monks will have interesting things to say.