Algorithm....

I'm working on a calendaring script right now, and am stuck on a programmatic way to figure something out. Basically, the work day is broken down into 15-minute intervals, where the value of a given interval can be figured as $hour=floor($i/4) and the minutes can be figured as $minutes=($i%4)*15. So 32 = 8:00, 33=8:15, etc. Currently, it's relatively easy to pull up a day, and block out those times that are already taken up with appointments. I'm throwing them all into an array ($available_times). So, that array would look like: 32=>yes 33=>yes 34=>yes 35=>yes 36=>no 37=>no 38=>yes 39=>yes etc, for someone who has a meeting from 9:00 to 9:30. Now, what I'm trying to do is check when they schedule a new meeting so that they don't overlap their times with something that's already taken up. So, in the array given above, if they select 8:30 as the start time, I'd like them to be able to select 8:45 or 9:00 as the end time, but nothing later than that, as the time from 9:00 to 9:30 is already taken up. And I'm stuck trying to figure out an approach to this. Preferably one that I can throw into javascript so they can fix their errors before it ever hits the server. Any help appreciated. Bob