Documentation better in commercial software?
Not in Microsoft Excel, according to Chip Pearson's The DATEDIF Function (emphasis mine):
The DATEDIF function is a worksheet function that computes the difference between two dates. This function is available in all versions of Excel since version 5, but is documented only in the help files for Excel 2000. It isn't documented in either Excel 97 or Excel 2002. DATEDIF has, for whatever reason, been treated as one of the drunk cousins of the Function Family. Excel knows he lives a happy and useful existence, and will acknowledge his existence when you ask, but will never mention him in "polite" conversation.
So the DATEDIF function has been in Excel since v5, and only ONCE since then placed in the help files - and not even for the latest version listed (2002)!
Very, very sloppy.