WeekYear

Weekyear, or week-based year, is the year associated with week-based date representations in calendar systems, most notably the ISO week date system. In this scheme weeks begin on Monday and week 1 is the week that contains the year’s first Thursday (equivalently the week that includes January 4). The weekyear of a given date is the year that contains that date’s ISO week, which can differ from the calendar year near year’s end and start. For example, December 29–31, 2014 belong to ISO week 01 of 2015, so their weekyear is 2015, even though the calendar year is 2014.

Computation and usage: Weekyear is used to group data by weeks across year boundaries and to label weekly intervals consistently. Many programming languages expose this value as a distinct component, such as week-based-year or weekYear. In Java, the API provides WeekFields.ISO.weekBasedYear (equivalently ChronoField.weekBasedYear); in Python, the isocalendar()[0] element gives the week-based year. Date formatting can also distinguish the week year from the calendar year; for example, some libraries use Y to represent the week-based year and y to represent the calendar year, which can lead to subtle differences around New Year.

See also: ISO 8601, ISO week date, week-based year.