Assuming cell A1(12/3/68) and B1(12/31/10). Another way would be that in the target cell enter =(B1-A1)/365. You can then format the target cell as a number with no decimal points and it will give you the same answer of 42. Either way gives you the same answer.

Excel stores the date as a number so you can use the same math with dates as you do with numbers. Format a cell as a number and type in a date. You will see how Excel stores it. It counts the days from 1/1/1900 to come up with the current date. For example today's date (8/12/10) is stored as 40402.

I hope this wasn't too much information, but it gives you an idea how Excel works with dates.