How to calculate Age of Debt in Days

Hi experts.

Basically this is my requirement. I need to produce a column in my query to display the age of debt in days.

I have inserted Characteristic Value Variable <b>Key Date, 0P_KEYDA</b> in my query. User will give an input to this variable and the report will be produced as at that date.

Now, there is a column called<b> Age of Debt in Days</b> in my query. The formula is as follow:

<i><b>Age of Debt in Days = Key Date - Document Date.</b></i>

I cannot define it earlier during data staging because the Key Date would be dynamic and I wouldn't know the value before the query is executed.

Can someone let me know on how can that be accomplished?

Thanks in advance.