This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I don't know why Sun didn't add that to the standard library. But calculating the difference between to dates is easy. You just look at the millisecond values of your two Date objects and subtract them - then you have the difference in milliseconds. It's straightforward to convert this to days (just divide by 24 * 60 * 60 * 1000, which is the number of milliseconds in one day).