Average based on date in previous column

Hi, I am trying to average numbers in a column, C, where the number is dependent on the date and time in the previous 2 column. I am having touble devoloping a formula to do this. Below is an example of what I am trying to explain.

A B C
11/10/2011 0:15 12/12/2011 1:16 46141
11/10/2011 0:16 12/19/2011 2:25 56289
12/1/2011 5:12 12/4/2011 7:37 4465
12/9/2011 0:22 1/20/2012 1:51 60569
12/13/2011 0:04 12/13/2011 0:18 14


Any help would be appreciated.