DATEDIF Function
http://www.cpearson.com/excel/datedif.aspx
Introduction To The DATEDIF Function
The DATEDIF function computes the difference between two dates in a variety of different intervals, such as the number of years, months, or days between the dates. This function is available in all versions of Excel since at least version 5/95, but is documented in the help file only for Excel 2000. For some reason, Microsoft has decided not to document this function in any other versions. DATEDIF is treated as the drunk cousin of the Formula family. Excel knows it lives a happy and useful life, but will not speak of it in polite conversation. Do not confuse the DATEDIF worksheet function with the DateDiff VBA function.
The syntax for DATEDIF is as follows:
=DATEDIF(Date1, Date2, Interval)
Where:
Date1 is the first date,
Date2 is the second date,
Interval is the interval type to return.