Does ISO Week Number upload my file or text to Tool-You servers?

No. ISO Week Number runs in this browser tab. Files stay on the device. Limit note: ISO-8601. Week 1 contains the first Thursday. Local YMD only.

Should I use Date Difference instead of ISO Week Number?

Use ISO Week Number when you need ISO-8601 week and ISO year. Use Date Difference when you need Count days, weeks, and calendar span between dates. They share a category (Date & time) but not the same job.

ISO Week Number

ISO-8601 week and ISO year.

Week 1 contains the year's first Thursday. Unlike Date Difference this is not a span. Unlike Calendar Quarter this is week-based, not month-based. Local midnight dates only.

What ISO Week Number does

Week 1 contains the year's first Thursday. Unlike Date Difference this is not a span. Unlike Calendar Quarter this is week-based, not month-based. Local midnight dates only.

ISO Week Number sits in the Date & time group on purpose. It is not Date Difference (Count days, weeks, and calendar span between dates.) and not Calendar Quarter (Q1–Q4 plus quarter start, end, and day count.). Choose this page when you need ISO-8601 week and ISO year. If you stretch ISO Week Number into a neighbor’s job, the result will look wrong even though the site chrome looks the same.

How to use ISO Week Number

Confirm you need ISO Week Number, not Date Difference (Count days, weeks, and calendar span between dates.). Address: /datetime/iso-week.
Enter the numbers or dates ISO Week Number uses. ISO-8601 week and ISO year.
Read the result as ISO Week Number defines it — not as Calendar Quarter (Q1–Q4 plus quarter start, end, and day count.). Week 1 contains the year's first Thursday. Unlike Date Difference this is not a span. Unlike Calendar Quarter this is week-based, not month-based. Local midnight dates only.
ISO Week Number is an estimate in the browser, not professional advice. ISO-8601. Week 1 contains the first Thursday. Local YMD only.
Need a different formula? Try Calendar Quarter (Q1–Q4 plus quarter start, end, and day count.) instead of stretching ISO Week Number.

When not to use ISO Week Number

Skip ISO Week Number when a neighbor tool matches better — start with Date Difference or Calendar Quarter — or when the description already warns the job will fail. In the tool’s own words: Week 1 contains the year's first Thursday. Unlike Date Difference this is not a span. Unlike Calendar Quarter this is week-based, not month-based. Local midnight dates only.

Questions about ISO Week Number

What does ISO Week Number do that other Date & time pages on this site do not?: Week 1 contains the year's first Thursday. Unlike Date Difference this is not a span. Unlike Calendar Quarter this is week-based, not month-based. Local midnight dates only. Short version: ISO-8601 week and ISO year.
Does ISO Week Number upload my file or text to Tool-You servers?: No. ISO Week Number runs in this browser tab. Files stay on the device. Limit note: ISO-8601. Week 1 contains the first Thursday. Local YMD only.
Should I use Date Difference instead of ISO Week Number?: Use ISO Week Number when you need ISO-8601 week and ISO year. Use Date Difference when you need Count days, weeks, and calendar span between dates. They share a category (Date & time) but not the same job.
What are the limits of ISO Week Number?: ISO-8601. Week 1 contains the first Thursday. Local YMD only. Also: Week 1 contains the year's first Thursday. Unlike Date Difference this is not a span. Unlike Calendar Quarter this is week-based, not month-based. Local midnight dates only.