ISO date weekday

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

ISO-8601 weekday number of a strict YYYY-MM-DD date (Monday=1 ... Sunday=7).

Use when you need to: iso date weekday · day of week iso · iso weekday number.

Supported

• iso date weekday
• day of week iso
• iso weekday number

Not supported

• locale weekday names
• Sunday-first
• timezones

Behavior

• Strict YYYY-MM-DD as above. Return weekday 1=Monday .. 7=Sunday.
• Do not return English or other locale names.
• Do not use local Date. Compute from a known civil epoch (proleptic Gregorian 0001-01-01 is Monday, ISO 1).

Input

• date (string, required): min length 10; max length 10

Output

• weekday (integer, required): min 1; max 7

Limits

• year min: 1
• year max: 9999

Example

Request input:

{
  "date": "1970-01-01"
}

Response:

{
  "result": {
    "weekday": 4
  }
}

How to call it

MCP

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

{
  "id": "iso-date-weekday",
  "version": "1.0.0",
  "input": {
    "date": "1970-01-01"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/iso-date-weekday/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"date":"1970-01-01"}'

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

CLI

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

Get the client at /clients/cli/.

Related tools

• ISO date day of year: Return the ordinal day number (1-366) of a YYYY-MM-DD date within its calendar year.
• ISO date from day of year: Convert a calendar year and ordinal day number (1-366) to a YYYY-MM-DD date.
• ISO date last weekday of month: Return the date of the last occurrence of an ISO weekday within a calendar month.
• ISO date month end: Return the last valid calendar day of the year-month of a YYYY-MM-DD date.
• ISO date month start: Return the first calendar day of the year-month of a YYYY-MM-DD date.
• ISO date nth weekday of month: Return the date of the nth occurrence of an ISO weekday within a calendar month.