How to find the difference between 2 dates?

My dataset aggregate value totals on a monthly basis. Each row represents an individual site and value for numerous metrics.

What I am trying to do would be how to find the change between 2 months like between Oct 1 , and Nov 1, as shown above for the values in the column to the right. But I am not sure how to write the appropriate beastmode.

The goal that I am trying to accomplish is to have a pivot table that I can take this column that would be showing the net difference of occupied area for the current and trailing 2 months.