Excel Formula - Calculating concurrent phone calls in a call center

Hello,
Hopefully someone can help me on this one. My goal is to:

1. Calculate how many concurrent calls a call center has at any given time within a 24 hour period.
2. In addition to item #1, be able to see how many concurrent calls at 15 minute intervals within a 24 hour period.

The information I have is (Columns A,B,C) = Date&Call end time/Call Duration (for ex: 2013-02-11, 23:16:15, 6.68)


I'm wondering if someone can help review my formula to check if I got this correctly. This is what I have so far to calculate how many concurrent calls at any given time within a 24 hour period:

=SUMPRODUCT(($A$1:$A$1000=A1)*($B$1:$B$1000=B1)*($C$1:$C$1000>0))