AverageIF and Offset Issue

I'm trying to make a formula that will calculate the average of the previous 12months. The goal is to tie the formula to a reference cell that contains a date. Each time the date is changed by a user the calculation will be updated accordingly. Here is the CSE formula that I thought would work:

{=AVERAGE(IF($B$5:$B$53=B$2,OFFSET($C$5,(COUNT(C5:$C$53)-1),0,12,1)))}

I've also attached a sample file to illustrate the problem. The 'range' portion ($C$5,(COUNT(C5:$C$53)-1)[/B]of the Offset function was setup simply to get the 12 months which preceeded the reference date. I'm reasonable sure this isn't the problem. Any help is appreciated.