Ole Begemann, in late 2011:

Pay special attention to the year format specifier @”yyyy”. It is different than the capitalized @YYYY, which represents the year of the date’s week and not the year of the day. 99% of the time, you probably want to use @”yyyy”. I have seen this bug so many times in production code that it’s not funny anymore so make sure your unit tests catch it.

It’s only not funny anymore until it is.