Use prompt value as within set in Aggregate function

[stevenbl]

I have a hard coded report that is comparing the values of two time periods.  I have been getting the two values using an aggregate function and them subtracting them.

The function is:
Aggregate([rank] within set [06/30/2012])

The request is to make the report more dynamic so that they can select the time periods to compare.  I created a prompt that would give me the date from my time dimension but I haven't been able to find a function to runt the parameter into something the within set function will take.

Any thoughts.  If there is a better way to compare two time periods please point me in the right direction.

Thank you,
Steven