Hello, I need help on a IF function or tell me if I can do this using a LookUp.

We have a spreadsheet to record the times patients arrive and the time they
are seen to by the doctor depending on whether they booked an appointment or
not. The Customer Service rules state that patients who have booked an
appointment have a 00:05:00 min waiting time. If this is True we type in "On
Time". IF False, we type in "Over Time".

However, IF they have NOT booked an appointment patients have a 00:15:00 min
waiting time. IF the "Seen At" time is less than 00:15:00 then it's "On
Time". IF False, then the value is "Over Time"

Here is an example of the spreadsheet we use:

Time arrived Appoinment Y/N Seen at Length of wait Ontime/OverT
15:00:00 Y 15:20:00 00:20:00 Overtime
15:15:00 N 15:35:00 00:20:00 Overtime
16:10:00 N 16:15:00 00:05:00 On Time


I would appreciate any clues and answers

Merry Christmas to all !!!
--
Regards

Irish Mike