Excel 2007 : Excel Help? Averages of Values over Variable Terms & Start Dates

I have a list of values that changes over time on a monthly or annual basis (for example June 2011 to May 2012 is $108, then June 2012 through May 2012 is $165, etc.) I need to be able to get an average of those values based on two inputs: term length in months and a start date. So, for example, I might have a 36 month term starting in September of 2011. How might I best go about organizing the data and building that formula?

Thanks!

bump?