MisusedWeekYear

"YYYY" in a date pattern means "week year". The week year is defined to begin at the beginning of the week that contains the year's first Thursday. For example, the week year 2015 began on Monday, December 29, 2014, since January 1, 2015, was on a Thursday.

"Week year" is intended to be used for week dates, e.g. "2015-W01-1", but is often mistakenly used for calendar dates, e.g. 2014-12-29, in which case the year may be incorrect during the last week of the year. If you are formatting anything other than a week date, you should use the year specifier "yyyy" instead.