|I'm certain a macro is needed|
There is a way you can "side-step" the =Today() function,
if your data is set up correctly.
In cell H3 is the Date of Service
In cell J3 is the number of Days
In cell L3 is the Date Order Closed.
Your current formula is =TODAY()-H3
If you modify your formula like this:
As long as the Date Order Closed is blank,
then you count days using the volatile =TODAY() function.
When you enter a Closing Date,
then we stop using the =TODAY() function and use the date in cell L3
which is your Order Closed Date.