| "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. |
| |
| This isn't an idle risk; Twitter had a |
| [significant outage](https://web.archive.org/web/20150711045621/http://tech.firstpost.com/news-analysis/twitter-suffers-massive-outage-on-all-online-platforms-back-now-247196.html) |
| in ~~2015~~ 2014 |
| [due to this bug](https://xcancel.com/jmhodges/status/549430032616017921). |
