ISO date day of year

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

Return the ordinal day number (1-366) of a YYYY-MM-DD date within its calendar year.

Use when you need to: iso date day of year · ordinal day of year · day number in year.

Supported

• iso date day of year
• ordinal day of year
• day number in year

Not supported

• ISO ordinal date strings
• timezones
• fiscal year

Behavior

• date is a strict YYYY-MM-DD proleptic Gregorian UTC calendar date with year 0001-9999.
• day_of_year is 1 for 1 January and 365 or 366 for 31 December, depending on whether the year is a leap year.

Input

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

Output

• day_of_year (integer, required): min 1; max 366

Limits

• year min: 1
• year max: 9999

Example

Request input:

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

Response:

{
  "result": {
    "day_of_year": 1
  }
}

How to call it

MCP

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

{
  "id": "iso-date-day-of-year",
  "version": "1.0.0",
  "input": {
    "date": "2026-01-01"
  }
}

HTTP (no key)

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

The machine-readable contract is at /v1/tools/iso-date-day-of-year/versions/1.0.0.

CLI

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

Get the client at /clients/cli/.

Related tools

• ISO date from day of year: Convert a calendar year and ordinal day number (1-366) to a YYYY-MM-DD date.
• ISO date weekday: ISO-8601 weekday number of a strict YYYY-MM-DD date (Monday=1 ... Sunday=7).
• 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.