Hi Hogen,
If the end goal is to use the values as a parameter, you would need to do this at the table-end itself, since you wont be able to create a parameter on a formula.
If there is already a logic that is counting the number of days past a certain Date, then why don't you simply write a stored procedure to 'insert' the Date of the Day back into the table. This will be helpful if the no. of days is varying.
A simple and better way is to have a Date column that is holding the required Dates from 1800/12/28 (or whatever) - till date.
-Prathamesh