Formula for Days Overdue if not Completed

I'm trying to format a worksheet to show how many days overdue certain tasks are, and running into a couple of issues. The sheet is set up with Due Dates in Column A, Days Overdue in Column B, and Date Completed in Column C. My first thought was just to use as simple a formula as possible in Column B =Today()-A2. This worked alright for tasks that already had Due Dates assigned, but if Column A was blank I'd get a high number in Column B. I also realized that tasks will continue to show as overdue even after I enter the completed-on date in Column C. Is there a relatively simple way to tackle this formula? Thanks in advance for suggestions. (I've attached an example with the column headings I want to use.)