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.