It sounds like you want a mechanism whereby you do some cleanup if it's been at least 'n' hours since the last cleanup, or if it's a new calendar date since the last cleanup, or if a cleanup's never been done.

It makes sense not to rely on cron if you don't know where the application is going to be rolled out -- I was only suggesting sticking to the cron format if there was a module that handles that already. Otherwise, go crazy and write your own protocol.

--t. alex
but my friends call me T.