ISO week to date

iso-week-to-date · version 1.0.0 · Dates & times (UTC, ISO 8601) · free, no key needed

Convert an ISO-8601 week date to a YYYY-MM-DD calendar date.

Use when you need to: iso week to date · convert iso week date to calendar date · calendar date from iso week.

Supported

• iso week to date
• convert iso week date to calendar date
• calendar date from iso week

Not supported

• locale week numbers
• Sunday-first weeks
• ISO ordinal dates
• timezones
• datetime output
• fiscal weeks

Behavior

• year, week, and weekday are JSON integers that pass Number.isInteger.
• year is 1-9999, week is 1-53, and weekday is 1-7 with Monday=1 and Sunday=7.
• Week 53 is rejected when that ISO year has only 52 weeks.
• Combinations whose calendar date would fall outside years 0001-9999 are rejected.
• Output date is a zero-padded YYYY-MM-DD calendar date.

Input

• year (integer, required): min 1; max 9999
• week (integer, required): min 1; max 53
• weekday (integer, required): min 1; max 7

Output

• date (string, required): min length 10; max length 10; pattern ^\d{4}-\d{2}-\d{2}$

Limits

• year min: 1
• year max: 9999
• week min: 1
• week max: 53
• weekday min: 1
• weekday max: 7

Example

Request input:

{
  "year": 2026,
  "week": 2,
  "weekday": 4
}

Response:

{
  "result": {
    "date": "2026-01-08"
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "iso-week-to-date",
  "version": "1.0.0",
  "input": {
    "year": 2026,
    "week": 2,
    "weekday": 4
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/iso-week-to-date/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"year":2026,"week":2,"weekday":4}'

The machine-readable contract is at /v1/tools/iso-week-to-date/versions/1.0.0.

CLI

node cli.mjs run iso-week-to-date 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

Related tools

• ISO date to week: Convert a YYYY-MM-DD calendar date to an ISO-8601 week date.
• ISO date add months: Add a signed month count to a YYYY-MM-DD date, clamping the day to the target month.
• ISO date add years: Add a signed whole-year delta to a YYYY-MM-DD date, clamping the day if 29 February has no match.
• ISO date from day of year: Convert a calendar year and ordinal day number (1-366) to a YYYY-MM-DD date.
• ISO date range split by month: Split an inclusive [start, end] date range into per-calendar-month sub-ranges.
• ISO date add: Add a signed whole-day delta to a strict YYYY-MM-DD calendar date.