Complex Aggregations - Looking for Help

I am working on a dashboard for a large retailer. This retailer has seasonal spikes and is looking for the ability to smooth out the bumps in their trend lines. My idea is for any individual month to look at that month and the X preceeding months and average the numbers over that time period. I've been trying to figure out how to sum X months of data under a single month.

For x=2, and the month of march 2013 this would mean, march 2013, february 2013, and January 2013 would all be summed together under the month of March. I am guessing I will need the help of some referential table that shows these one to many relationships but I am not sure how I would need to structure the query to get this to work.

Please advise if you have any ideas,

Thanks!

Is this in Access or Excel? Access is just serving as a database and I'm tapping it with excel. If excel has this functionality I will use it but how would i use access to do this for the frontend?

It is mostly chart based but these charts are also paired with tables for numeric data. I looked at what excel has for this (and it is kinda nifty) but doesnt quite meet my needs as it cannot use data from outside the shown window to calculate rolling numbers. I am trying to show a 13 month window with each of those 13 months being the average of the 12 before it. Let me know if that makes sense.

Is there a way using a query to do that in access?