Week Number Calculator API

ISO 8601 week number, ISO week date, day of year and quarter for a date, or the Monday–Sunday dates of a given ISO week.

POST /api/v1/tools/week-number-calculatorMCP week_number_calculatorTime & Date

Find the ISO 8601 week number of any date, plus day of year, days left, quarter and leap year, or look up the Monday-to-Sunday dates of any week.

Input

Send these as a JSON object. Unknown fields are rejected.

date: A date as YYYY-MM-DD to get its ISO week. Give this, or year and week. (string, up to 10 characters)
year: ISO week-numbering year, to get the dates of a week (with week). (integer, 1 to 9999)
week: ISO week number 1 to 52 (or 53 in long years), with year. (integer, 1 to 53)

Try it in the browser JSON Schema Get an API key

curl https://pithlab.com/api/v1/tools/week-number-calculator \n  -H "Authorization: Bearer $PITHLAB_KEY" \n  -H "Content-Type: application/json" \n  -d '{"date":"2026-10-20"}'

Example response

{
  "ok": true,
  "result": {
    "week": 43,
    "weekYear": 2026,
    "isoWeekDate": "2026-W43-2",
    "weekday": "Tuesday",
    "dayOfYear": 293,
    "daysInYear": 365,
    "daysRemaining": 72,
    "quarter": 4,
    "isLeapYear": false,
    "weeksInWeekYear": 53,
    "weekStart": "2026-10-19",
    "weekEnd": "2026-10-25",
    "summary": "Week 43 of 2026 (2026-W43-2), day 293 of 365, Q4"
  }
}