Date Arithmetic in Swift

Since Manifest is a time tracker, it does a fair amount of date arithmetic. Cocoa has excellent support for creating and manipulating dates, but it's not a very lightweight API. To make things easier, I wrote a small Swift wrapper that lets you do things like this:

let tomorrow = NSDate() +
let lastYear = NSDate() - 1.year()

I've released it as a small open source project, which you can grab on GitHub. More detailed documentation is available there as well. Suggestions and feedback welcome!