ISO date to week

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

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

Use when you need to: iso date to week · convert calendar date to iso week date · iso week year week and weekday.

Supported

• iso date to week
• convert calendar date to iso week date
• iso week year week and weekday

Not supported

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

Behavior

• Input date is a strict YYYY-MM-DD proleptic Gregorian UTC calendar date with year 0001-9999.
• ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of the ISO week-numbering year.
• The ISO week-numbering year may differ from the calendar year near 1 January.
• Output year, week, and weekday are JSON integers. weekday is 1=Monday through 7=Sunday.
• iso is YYYY-Www-D with a zero-padded year and week (for example 2026-W02-4).

Input

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

Output

• year (integer, required): min 1; max 9999
• week (integer, required): min 1; max 53
• weekday (integer, required): min 1; max 7
• iso (string, required): pattern ^\d{4}-W\d{2}-[1-7]$

Limits

• year min: 1
• year max: 9999
• date length: 10

Example

Request input:

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

Response:

{
  "result": {
    "year": 2026,
    "week": 2,
    "weekday": 4,
    "iso": "2026-W02-4"
  }
}

How to call it

MCP

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

{
  "id": "iso-date-to-week",
  "version": "1.0.0",
  "input": {
    "date": "2026-01-08"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/iso-date-to-week/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"date":"2026-01-08"}'

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

CLI

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

Get the client at /clients/cli/.

Related tools

• ISO week to date: Convert an ISO-8601 week date to a YYYY-MM-DD calendar 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 business day add: Add a signed count of business days to a date, skipping caller-supplied weekend weekdays and holidays.
• ISO date from day of year: Convert a calendar year and ordinal day number (1-366) to a YYYY-MM-DD date.
• ISO date quarter bounds: Return the calendar quarter number and the first/last dates of the quarter containing a date.