What does this tool do?
ISO 8601 weeks begin on Monday and week 1 is the week that contains 4 January. Around New Year the ISO year can therefore be one year ahead of or behind the calendar year.
The Formula
ISO week = floor((day of year − ISO weekday + 10) ÷ 7), with week 1 containing 4 JanuaryFrequently Asked Questions
Why does the ISO year sometimes differ from the calendar year?
Late December dates can fall in week 1 of the next ISO year, and early January dates can belong to the final week of the previous ISO year. The week is assigned by the Thursday it contains.
Where is the ISO week date used?
It is common in European payroll, scheduling standards, manufacturing and data exchange, where a stable Monday-based week numbering matters.