Primavera Analytics - Is it possible to calculate a delta between two monthly historical values?

I have an analysis which which consists of a pivot table showing two columns for monthly historical date fields (BP Counts in one case, Early Start dates in another case) from the Activity History subject area, and I wish to calculate the difference between the two. 

While it is fairly straightforward to calculate a delta between a current value for an activity and a value from the history table, I am unable to see any way to calculate the difference between the two historical values as the two columns are both identified as "Month".   I've tried using a presentation variable in the analysis which names the columns based upon a user's input, and while this works if I provide default values for the presentation variables it fails when I take the default values out and use a prompt to fill them.