Date calculations

Hi,

on my work I often receive documents with a due response date (that is already late when I recieve it…).

Anyway, I created a spreadsheet to control witch responses are more or less late to organize my work.

The spreadsheet has a column with the due date and another wich is "=today()- the value on the due date cell).

It´s good, but I would like it to stop counting when I get done with that process.

How would I do that in a simple way?

How can the cell conatining your formula =TODAY()-A23 (A23 containing the due date) know that you got done with that process?
If there is a cell, say C23, where you enter something like “Done!” or the date of your response or whatever when finished with the issue, you may use the formula =IF(C23="";TODAY()-A23;"") instead of the above mentioned one.