| "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. |