I would probably look at doing it with a more elaborate data structure, like a tied DBM hash, just because my guess would be that that will scale better over the long run.

The other thing I would do if I were you is have a look over at Freshmeat; I know there are similar web calendar systems over there that you could probably either (a) adapt to your purposes or (b) scavenge code or ideas from.

Good luck!
john.