Query Based Element

I am trying  track contract completion dates. 

The current formula is   Original Completion Date + Days Changed per change orders = Revised Completion Date.  The Original Completion Date and Revised Completion Date are Date Pickers.  Days changed per change orders is  entered manually.

I created a summary element for Days Changed per change orders, but can't use that in a formula.  So, I created a query based data element to do the math.  But when I try to define the qbde, I can't add dates.

Any suggestions?