An interesting one!!!

Here is a puzzle, infact it is a game that I need to do in PHP, here is the spec 3 prizes to be won every day over a month by clicking on 24 seperate windows....and thats it How could I go about deciding whether a user that clicked on one of the windows is a winner or not, obviously I wouln't want the prizes to go in the first hour of the day, so would need to spread it out abit. Any ideas would be greatly appreciated????? Cheers, Tim.