A week date is a type of calendar which uses the year, week of the year, and day of the week to refer to to dates. In the ISO 8601 week date, the first week of the year is the week starting from Monday which contains January 4th. An ISO 8601 week date can thus have a different year number than the corresponding Gregorian date. The first week of the year is numbered 01 , and the first day of the week, Monday, is numbered 1 . Weeks are written in YYYY-Www-D or YYYYWwwD format, where the upper case W is literal. The corresponding strftime literals are %G-W%V-%u and %GW%V%u .