Hello,   I have the following problem, for which I have been unable to find a solution through searching online.   Lets suppose we have the following table ("Purchases"):   Date Units_Sold Brand Year 18/03/2010 5 A 2010 12/04/2010 2 A 2010 22/05/2010 1 A 2010 25/05/2010 7 A 2010 11/08/2011 5 A 2011 12/07/2010 2 B 2010 22/10/2010 1 B 2010 05/05/2011 7 B 2011   And the same logic continues until the end of 2014, for different brands. What I want to do is calculate the number of Units_Sold for every Brand, in each year. However, I don't want to do it for the calendar year, but for the full year after a Brand appears (365 days).   So an example of what I don't want:   proc sql; create table Dont_Want as select Year, Brand, sum(Units_Sold) as Unit_per_Year from Purchases group by Year, Brand; quit;   The above logic is ok if we know that e.g. Brand "A" exists throughout the whole 2010. But if Brand "A" appeared on 18/03/2010 for the first time, and exists until now, then a comparison of Years 2010 and 2011 would not be good enough since for 2010 we are "missing" 3 months.   So what I want to do is calculate the sum of Units_Sold of a full year of a Brand since its first appearance on the data-set. For instance:   for A: the sum from 18/03/2010 until 17/03/2011, then from 18/03/2011 until 17/03/2012, etc. for B: the sum from 12/07/2010 until 11/07/2011, etc. and so on for all Brands.    Is there a smart way of doing this?   Thanks, Fil ... View more