Calculate the first release dates of all products in a database so they can be align and first weeks

Hi all

I managed to build a model from other posts that enables me to compare the sales of products launched at different release dates by aligning the first week sales etc. However, I have to manually enter the release dates into a separate table. If I try and do it dynamically, using Firstdate function it always seems to default to the first date for any transaction in the database.

http://www.powerpivotpro.com/2014/06/compare-product-performance-after-launch/

Any my thoughts would be gratefully accepted.

Thanks

Matt