Also when using a date format string using the correct format is important.
@”YYYY” is week-based calendar year.
@”yyyy” is ordinary calendar year.
You can go through the whole blog, its a good to give it a look
https://web.archive.org/web/20150423093107/http://realmacsoftware.com/blog/working-with-date-and-time
http://realmacsoftware.com/blog/working-with-date-and-time (dead link)