WeekFieldsISOweekBasedYear

WeekFieldsISOweekBasedYear refers to the ISO week-based year as used in the ISO-8601 week date system. In the Java time API, it is exposed as the ISO-defined week-based year field, available through WeekFields.ISO.weekBasedYear() (a TemporalField). The week-based year identifies the year associated with a date when it is expressed in ISO week-date terms.

The ISO week date system defines weeks that start on Monday and assigns week numbers so that Week 1 is the week containing January 4. This means the ISO week-based year can differ from the calendar year for dates near year boundaries. For example, 2012-01-01 falls in week 52 of 2011, so its ISO week-based year is 2011. Conversely, the last days of December can belong to the first ISO week of the following year; for instance, 2014-12-29 through 2014-12-31 belong to week 1 of 2015, giving those dates an ISO week-based year of 2015.

Purpose and usage: Week-based year is used when working with ISO week dates, allowing calculations and formatting that align with week-centric representations of time. It is commonly used in business calendars, fiscal planning, and any domain where weeks—not calendar years—define periods.

In practice, you typically pair the week-based year with WeekFields.ISO.weekOfWeekBasedYear() to obtain a complete ISO week date, or use them in formatting and parsing routines that rely on ISO conventions. See also WeekFields.ISO, WEEK_OF_WEEK_BASED_YEAR, and WEEK_BASED_YEAR in the API.