Difference between two columns???

Hi ,

I am developing a WebI report. I get date value as 'AsOnDate' from table. Also position from table as 'Achievement'.

My requirement is to compare and show the business position on the two dates and to show the difference for a number of items. What I have done is added two input controls for 'From' and 'To' date. I have two blocks in the report with 'Achievement' in each. One input control affects one block and the other affects the other block. Till there it works fine.

But the problem is how to display the difference of business position,by subtracting the From Date 'Achievement' from To Date 'Achievement'.

Thanks in advance

Sandeep