ISO week date
About the ISO week date
The ISO week date in ISO 8601, first published in 1988, numbers weeks from Monday to Sunday. Week 1 is the week containing the year's first Thursday, so it always includes 4 January. A year has 52 or 53 weeks, and 71 years in every 400-year cycle have 53. Dates are written like 2026-W41-1, and the ISO week-year can differ from the calendar year around New Year.
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