PowerPivot DAX case: compare product sales by months since product introduction

A few weeks ago I got this interesting question from someone. He wants to compare the sales growth of his products in the months since the introduction of the product. Lets say "Product One" first selling date is 1/2/2010 and "Product Two's" first selling date is 2/1/2010. If we want to compare the sales since introduction we want to compare month 0 between the two products. Month 0 for Product One is 1/2/2010, for Product Two it is 2/1/2010.