Recording 12 Month Revenue

Hello.

I'm looking for a way to find a customer's revenue prior to joining a different line of business in my firm. Sheet 1 has the customer's start date and sheet 2 has all of this customer's ID and revenue from 2014 to May 2016. This customer started in January 2015 so I would need to return the sum of the 12 months prior to January 2015 (not including January). Any help would be great!