# ISO Week Number

Converts calendar dates to ISO 8601 week-numbering dates and back. Gets right the part every ad-hoc script gets wrong: week 1 is the week containing the first Thursday of the year, so late-December and early-January dates routinely belong to a different ISO year than the calendar year.

**Not for:** Not a general calendar or scheduling tool, and it does not know about business fiscal-year or retail 4-4-5 week numbering, which are different systems that also call themselves "week numbers." It has no concept of time of day or timezone — it operates on calendar dates only. Not for US-style "week of the month" or non-ISO week-start conventions (e.g. Sunday-start week numbering used by some US calendars).

## Surfaces

- HTML: `https://iso-week.gumballtools.com`
- Markdown: same URLs with `Accept: text/markdown` or `?format=md`
- JSON API: `https://iso-week.gumballtools.com/api/v1/run`
- OpenAPI: `https://iso-week.gumballtools.com/.well-known/openapi.json`
- MCP: `https://iso-week.gumballtools.com/api/mcp`