difference from prior period in pivot tables

How do I difference from same item prior period in a pivot table. I went to summarize values and they have difference from previous, next and base item but nothing that compares 12 months back.

I know you can group row lables, but that won't work in this case because the row labels aren't really dates but are formatted like m01,m02,mo3, etc., so I would want to compare difference from month 1 to prior year month 1. Capisce?