weekbasedyear

A week-based year is the year used by the ISO week date system to label weeks. In this system, each date is assigned a week number and a week-based year. Weeks begin on Monday, and week 1 is defined as the week that contains the year's first Thursday (equivalently the week that contains January 4). The week-based year is designed so that weeks are grouped consistently across year boundaries, which means some days at the end of December may belong to the final week of the current year while others at the beginning of January belong to the first week of the next year.

The concept is standardized in ISO 8601 and is widely used for international business, scheduling, payroll, and reporting. Many programming languages expose the ISO week year as part of their date APIs; for example, functions or methods that return isocalendar components deliver the ISO year, week number, and weekday, reflecting the week-based year rather than the calendar year.

A year in the ISO system can have 52 or 53 weeks. A 53-week ISO year occurs when the year starts on a Thursday in a common year or when a leap year starts on a Wednesday. This can cause dates at the end of December to fall in week 1 of the following week-based year.

In practice, the week-based year offers a stable framework for week-centered planning and reporting, independent of irregularities at the calendar year’s edges.