I want to calculate the ISO year number based on a date. First ISO week: The ISO 8601 definition for week 01 is the week with the Gregorian year's first Thursday in it. The following definitions based on properties of this week are mutually equivalent, since the ISO week starts with Monday: It is the first week with a majority (4 or more) of its days in January. Its first day is the Monday nearest to 1 January. It has 4 January in it. Hence the earliest possible first week extends from Monday 29 December (previous Gregorian year) to Sunday 4 January, the latest possible first week extends from Monday 4 January to Sunday 10 January. It has the year's first working day in it, if Saturdays, Sundays and 1 January are not working days. ISO YEAR ISO WEEK Starts on 2019 1 ma 31-12-2018 2020 1 ma 30-12-2019 2021 1 ma 04-01-2021 2022 1 ma 03-01-2022 2023 1 ma 02-01-2023 2024 1 ma 01-01-2024 2025 1 ma 30-12-2024 2026 1 ma 29-12-2025 If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, it is part of week 53 of the previous year. If it is on a Saturday, it is part of the last week of the previous year which is numbered 52 in a common year and 53 in a leap year. If it is on a Sunday, it is part of week 52 of the previous year. ... View more