Given an end date find corresponding start date 3 months prior ..quarter date range

hi,

I am trying to find / determine a start date based on an end date. The end date is always the end of a given month.

Then what i want to do is go back 3 months to the begining of the 3rd month (see below):

Start : End
9/1/11 : 11/30/11
8/1/11 : 10/31/11
7/1/11 : 9/30/11
6/1/11 : 8/31/11


So you would be given the end date above and need to figure outthe corresponding start date............

Anyone have any ideas?

Or, without the analysis toolpak, or XL2007

=DATE(YEAR(A1),MONTH(A1)-2,1)