Excel Formula Help please!?

I am working on a spreadsheet for work and its is for billing purposes. I want to be able to have excel calculate the total minutes between start and stop times for appointments but is there a way it can only count 15 minute increments? For example- Start Time: 1:30 Stop Time: 2:30 which is 60 minutes in between. At my work, we bill "ONE UNIT PER 15 minutes" so that 60 minutes equals 3 units. BUT here is my dilemma, excel is counting the minutes in between the two times just fine but when i divide by 15 it does not round down it rounds up. We cannot bill that way, so even if an appointment time goes over a few minutes but doesn't hit a 15 minute increment, we do not count it as a unit. So what i am looking for is a formula that counts every 15 minutes between times OR a way to get the total of minutes in between to equal multiples of 15 minutes. THANK YOU SO MUCH to anyone who can help!!....i know this may be a little confusing