Average intervals by range of dates

[ptrifile]

I have a report that i need to get averages by 15 minute intervals by a range of dates. I currently have the 15 minute intervals but I cannot seem to get an average of all the dates. My report looks like this:

Date calls
9/16/2013 8:00am 2
9/16/2013 8:15am 3
9/16/2013 8:30am 1
9/17/2013 8:00am 4
9/17/2013 8:15am 6
9/17/2013 8:30am 3

I would like it to look like this:
Date calls
8:00am 3
8:15am 4.5
8:30am 2

Can someone give me some suggestions on what i need to do to get the desired results?

Thank you in advance for any help.

Thanks!

Paul