The DATEDIF worksheet function
The DATEDIF function calculates the difference between two dates, and expresses the result in terms of months, days, or years.
If you have Excel version 5.0 or later, you can make use of the handy DATEDIF function in your formulas. But if you want to read about this function in the online help, you must have Excel 2000. For some reason, the function was never documented until Excel 2000. And the documentation was removed from Excel 2002.
The DATEDIF function has its origins in Lotus 1-2-3. Apparently, Microsoft included it in Excel for compatibility purposes. Why is it not documented? Who knows? But it's likely that lawyers are involved.
Chip Pearson has some good information about DATEDIF at his web site.