Hi - I want to count based on a rolling data range and I can't seem to figure
out how to do it.

My data looks something like this:

Jan09 Feb09 Mar09 Apr09................ Jan10 Feb10 Mar10 Apr10
15 25 1 4 7 15
9 28

I want to count the numbers based on 12 months of data. So if Apr10 has a
number in it, I want to count from Apr10 all the way back 11 months through
May09.

If Apr10 was blank then I would like the formula to pick up from Mar10
(assuming it has a number in it) all the way back through Apr09

Is this possible?