Windows 7 64bit / MS Excel 2007

I need to create a simple weekly timesheet with the following data and calculation;

A 'time in' cell "A", a 'time out cell' "B" for each day of the week, and a calculation cell "C" for the hours worked each day.

I thought I was pretty smart till I tried to do this simple task, but . . . . I'm getting old :-) Your help is very much appreciated - IM