Calendar desk
ISO Week Number
Find the ISO 8601 week and ISO year for a date, including year-boundary cases.
ISO Week Number: Identify the ISO week number and week-numbering year for a date. Weeks run from Monday to Sunday, and week 1 contains the year's first Thursday. Near New Year, a January date can belong to the preceding ISO year, or a December date to the following one. Runs 100% locally in your browser with zero server file uploads.
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ISO week
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- ISO year
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ISO week = week containing Thursday; weeks start Monday and week 1 contains January 4- Dates are calendar dates at UTC midnight; no local timezone or daylight-saving adjustment is applied.
- Results are planning utilities. Public holidays, locale-specific work rules, and company policies are not inferred.
- ISO weeks start on Monday; dates near New Year can belong to the prior or next ISO year.
ISO 8601 rule and boundary example
ISO's public overview (https://www.iso.org/iso-8601-date-and-time-format.html) introduces the date-and-time standard. The public Python calendar summary (https://docs.python.org/3/library/datetime.html#datetime.date.isocalendar) explains its Monday start and first-Thursday week rule without requiring a copy of the paid standard.
Let M be the Monday of ISO week 1 in the relevant ISO year, and D the day number of the input. Week W = 1 + floor((D − M) ÷ 7). Determine the ISO year from the Thursday in the input's week, rather than simply taking the input's calendar year. This keeps all seven days assigned to one week-year pair.
For 2021-01-01, its week's Thursday is 31 December 2020, so the ISO year is 2020. Week 1 of that year began on 30 December 2019. The input is 368 days after that Monday: W = 1 + floor(368 ÷ 7) = 53. The widget returns week 53 and ISO year 2020.
Using a complete week label
An ISO year has 52 or 53 complete weeks. The leap-year flag alone does not tell you the week count; the position of weekdays at the year boundary also matters. A bare label such as week 1 is incomplete for a schedule spanning several years. Record the returned ISO year with the number and include actual Monday and Sunday dates when colleagues use different calendars.
How to use it
- Enter the date that needs an ISO week label.
- Calculate its week number.
- Keep the ISO year alongside the week number, especially around New Year.
Privacy & limitations
Dates, times and results stay in your browser. No inputs are uploaded.
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Frequently asked questions
Why can a January date show the previous ISO year?
A week stays together from Monday through Sunday. If those early January days are in the last week assigned to the previous year, the result uses that previous ISO year even though the calendar year has changed.
Does ISO week one always begin on 1 January?
No. Week 1 is the Monday-to-Sunday week containing the first Thursday, equivalently 4 January. Its Monday can lie in late December or in the first few days of January.
Is a Sunday-first week number interchangeable with this result?
No. Other systems can use a different first weekday and a different rule for week 1. When comparing schedules, confirm ISO numbering and share the week-numbering year as well as the number.
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