Percentage between 2 dates in Excel

I need a formula to give me the percentage between to dates. If I give an employee a completion date for his work I want to be able to compare those 2 dates Plus or minus 10%. So if I give them a 100 hour project with a due date of 30-Apr-11 and then complete the work by 15-Apr-11 they have completed it early. But in a percentage how much early did they complete it in?

I have completed this in hours using "=(O12-N12)/N12"
But when I copy this and use it for dates * an always receiving a value of ZERO returned.

Thanks