Need to determine the number of years between two dates.

12/03/1968 - 12/31/2010

Asumming the dates are in cells A1 and A2 respectively, you can use the following formula:

=YEAR (A2)-YEAR(A1)

That solution gives you one whole year between Dec. 31st and Janury 1st even though there is only one day between them, so perhaps you should also try:
NSV

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.