From his inital blog entry about it:

"I saw Matt Drake’s post with tips about using NSDate over on Mobile Orchard’s iPhoneFlow today and figured I’d give the drummer some more.

When setting up Codebook (a secure notebook iPhone app we hope to release soon) I needed to do some fancy-pants date formatting all over the place because I wanted to mimic what Apple had pulled off in the Notes application for iPhone. Now, there are many ways to do this, I’m sure I’ve overlooked some useful tricks that Cocoa provides (it has a number of default date format “styles” you can use), but I think what’s important is to always avoid repeating yourself. And to do it in such a way that if, say, I realize that I really should have used the Cocoa candies, I could change the implementation in one place and the rest of my code would be none the wiser. DRY, as it were.

Now date calculations in Cocoa can be nasty. I don’t know if they have to be, but you can end up with a real mess of code involving NSCalendar, NSDateFormatter objects, or calculating offsets by seconds. Totally ugly.

Objective-C’s categories are a really nice way to bottle up some of this code and re-use it. In Codebook I used categories to implement helper methods on built-in objects and classes, like NSDate. So in the aforementioned example of Codebook, I implemented a series of methods for doing date calculations, and convenient methods for getting calculation-based formatted dates."