Normalizing to neutral time period
 

I have cutomer data in rows for the past 24 months in two seperate spreadsheets.

The first spreadsheet shows the first time that a customer purchased product A.
The second spreadsheet shows revenue for the same time period for product B.

I'm trying to build a chart that shows average spend on product B for the 6 months prior to purchasing A and the 6 months following.

The problem is I don't know how to convert the data so that the first purchase of product A is always the same point of time on the chart.