Microsoft says that Windows Blue (Windows 8.1) will be free for existing Windows 8 users. See the article on ZDnet.
Between the Sheets — Excel podcast episode 31: Excel’s secret DATEDIF function
A quick and simple way of finding the number of days, months or years between two dates is Excel’s secret DATEDIF function. When you type it in a worksheet, there’s no popup syntax help to indicate the function exists.