This has (sort of anyway) been discussed a number of times before. Please search the forums for info on 'crontab' and such.
However, mysql does not (that I know of, I'm not activly watching) have triggers like postgresql. Using a trigger you could make the db itself call a script whenever an entry outdates or whatever. This is (like I said, to my knowledge) not possible in mysql. You have to write a script outside of mysql and launch it using other programs (crontab is nice).