Excel 2007 : Stopping a date count in Excel

I am a virgin here so please be kind...Here is what I have.

I have a spreadsheet that tracks invoices.
Column B is my invoice date
Column C is the due date
Column F is how many days the invoice has been active
Column M is the date the invoice was paid.

Column B utilizes a formula to give me a due date in Column C and is working fine

Column F uses =NOW()-C to give me the number of days the invoice has been in the system

What I am trying to do is to stop the day count in Column F once I have a date entered in Column M.

I would appreciate any help. Please let me know if you have any questions.

Thanks!