I would like to prevent the overlap of appointments in ol2007. I have published a custom form to a public calendar that will allow others to create appointments for usage of a computer. All I would like to do is make sure that no two appointments occupy the same time.

Example:
appointment1; 12:00-14:30
appointment2; 13:00-18:00

If someone tried to create appointment2, it wouldn't be allowed on the public calendar. If appointment1 ended at 13:00 or appointment2 began at 14:30, there would be no issue in creating the appointment.

p.s.- I have searched. Nothing useful found. Also, the internet here is s - l - o - w, so a basic search takes at least 5 minutes.