Formula for a Rolling 12 month Average

[dandavis1]

See Attached.

The YTD 10 column is fine that will change everytime you update each month. I was wondering if there was a formula I could use for the Rolling 12 month Column.

At the moment it is averging from Mar 09-Feb10 which is correct but when it comes to putting in data for Mar 10, the Rolling 12 month column will obviously not change.

Is there a way of moving the formula along one each time you input data into a new month. So when Mar 10 has been entered the rolling 12 month column will pick up Apr-09-Mar-10 then when Apr 10 has been entered the formula will change again to May-09-APr-10?

Many Thanks....