Very simple example of Data I'm working with:

Month Store Sales
1/31/2007 west 400
2/28/2007 west 100
3/31/2007 west 300
4/30/2007 west 500
5/31/2007 west 400
6/30/2007 west 100
7/31/2007 west 300
1/31/2007 east 400
2/28/2007 east 100
3/31/2007 east 300
4/30/2007 east 500
5/31/2007 east 400
6/30/2007 east 100
7/31/2007 east 300

In my formula, I will know the store and I will know the Date. Say east and
5/31/2007. In that case I will want to sum the sales for east for may,
april, and march. (thus the trailing three month sales for east as of may)

In reality I have more criteria columns and need to do trailing 12 months or
more, but a solution to the simple example above will show me the way. I
have built mega Sumproduct formulas where I use date functions to count back
months and add all the results, but the formula's are so long they sometimes
don't fit in a cell.

Any ideas? If it helps I can nest an offset into the formula that contains
the values of may april and march from a seperate table of dates, but I
havn't figured out how to use that to my advantage or if that is even the
right approach.