|I'm assuming the due date is in E2.|
This will return the difference between Today's date and the date in E2:
Nested within an IF function, it could be used like this:
=IF(DATEDIF(E2,TODAY(),"d") >13, "The item is overdue", "The item is not overdue")
Note: The Help file for DATEDIF is missing in Excel 2003. If you want to learn more about it, look here:
Click Here Before Posting Data or VBA Code ---> How To Post Data or Code.