use of countif formula to calculate time intervals eg.15 min interval

I have data with dates and time slots where in the calls have hits. I need to annalyse call intervals by 15 mins gap in between.
eg. from 5:00 to 5:15, 5:15 to 5:30 and there on...

please help me with the appropriate formula to calculate time interval from the time range...



thanks
Sachin

It will be count if, with two criteria, Start>=min time and End<=max time