How to calculate Offset Periods

[cbutcher]

I Need a little help

In the attached application I need the Calc2 column to calculate the Current selection by month + the offset value i.e.

If I select 201001, 201002 and 201003 (April2010, May2010 and June2010)

I need the calc2 to be April2010 + May2009

May2010 + June2009

June2010 + July2009

The calculations are there for both in separate columns but I can't get it to calculate in the same column against the currently selected Periods.

Any pointers would be great

Thanks

Chris