schedule events

Hi everybody, My question is pretty easy, so i hope the answer too :) Here it comes, I am programming an online schedule, and I need to know at a certain moment, how many events can occur, for instance, how many tasks to be done at 9:00 am, with the problem that users can insert tasks selecting hour and minute of start and end time, for instance, item1 takes from 8:00 to 9:50, item 2 from 8:45 to 9:10, so at 9:00 it should return 2 items concurring... Thanks in advance for any help, josemaria sala