Sorry guys for saying this a little late, but this change makes this method thread-unsafe. Because `NSDateFormatter` [is not thread safe](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/ThreadSafetySummary.html), but `NSDate` is thread safe. Calling this method from different thread at the same time might cause an issue, because from the both threads the same `NSDateFormatter` object will be accessed.

Personally I use https://github.com/mysterioustrousers/MTDates for date calculations and this problem is solved there using locks. You could take a look at the implementation there.