Combining the IF Function with a countdown to show a project "Completed"

Hello all, thanks in advance for responding to my first post.

I'm designing a spreadsheet for use by my law firm, and the idea is to track due dates and show when things are filed or completed.

We have three cells being used in this case - a "due date" cell, a "filed date" cell and a "days until due" cell. I have a countdown setup so that the days until due cell currently shows how many days are left until the item is due - which is simply:

=U5-$A$1

Where U is the "due date" column, and A1 is today's date.

What I can't figure out how to do is make the "days until due" cell show up as filed when a date is inputted into the "filed date" cell.

So for example, let's say the due date is September 15, 2013 and the days until due counter is showing that we have six days until it's due. I would like that same cell to say "FILED" as soon as someone inputs the date it was filed into a separate cell.

Thanks again...

My faith in humanity has been restored! Thank you TMS.