Formatting Years as Strings

In date formatting, yyyy is not the same as YYYY.

Apple’s Date Formatting document describes this:

A common mistake is to use YYYY. yyyy specifies the calendar year whereas YYYY specifies the year (of “Week of Year”), used in the ISO year-week calendar. In most cases, yyyy and YYYY yield the same number, however they may be different. Typically you should use the calendar year.

You can test Swift’s DateFormatter with 2016-01-01: YYYY will tell you it’s 2015, not 2016, because it’s a Friday, at the end of 2015 by the year-week calendar logic.

If you’re working with date formatters, it’s a good idea to include 2016-01-01 in your unit tests, just to make sure you format the date as you expect.