I'm using Microsoft Office 365.
I've discovered the DATEDIF formula in this forum, but it's not to working for me.
I've tried several variations and get the #NAME? error.
The first two arguments must be dates: references to cells containing dates, or expressions that evaluate to dates.
The third argument must be a string: text enclosed in quotes, or a reference to a cell containing a string, or an expression that evaluates to a string.