The week-based year is the year component of an ISO Week Date. It differs from the calendar year for late-December or early-January dates that fall in an ISO week belonging to an adjacent year.
Why it differs
ISO 8601 assigns every week entirely to one year, even when the week straddles 1 January. The week-based year is the year that owns the week, determined by the First-Thursday Rule.
Worked example: 1 January 2023 was a Sunday. The ISO week containing that Sunday started Monday 26 December 2022 and contained only 1 day in 2023. Its first Thursday was 29 December 2022. The week therefore belongs to 2022. The ISO week date for 1 January 2023 is 2022-W52-7 — week-based year 2022, even though the calendar year is 2023.
Calendar year vs week-based year
The 2 values agree for most of the year. They diverge only for dates within the first 3 days of January or the last 3 days of December. Software that mixes the two without conversion produces off-by-one-year reporting errors.