I have a pivot table and in the data area that I want to base the
calculations off other rows in the data area. However, when type the = sign I
get a message that says "Cannot change part of a PivotTable report." Is
there any way around this.

To give you an idea I have a field called Allocated Space in the Row area
and in the Column area I have Date. The field I want to calculate is called
"Allocated Space Diffrence" and it's calculated as Allocated for date
selected - Previous month's Allocated. (i.e. 10/1 Allocated Space - 9/1
Allocated Space, 9/1 Allocated Space -8/1 Allocated space, etc)