How to create a filter that is based on another dimension with different timeframe for each row

[eliskac]

Hi all!
I have a list of stores and each started supplying a different date. I need to calculated what was their average supply for the first 28 days since they started supplying. The first supplying date differs for each store. 

My idea is that I have a table, rows is "store name" dimension". And then columns are supply per each day, I would create table calculation for the average supply. And it would be pivoted by date. I am able to create this until here. The thing is that I only need to show the results for the first 28 days since they started supplying. And I thought I could have two "date" filters, one would be "date" - where I would choose for instance "any time" or "this year" and the second "date based" filter would be the "first supplying date" and here I would somehow need to create that it only filters the first 28 days. Do you know how to do this via custom filter? And do you think this is a good solution or do you have other ideas?

Thanks!