ISO date from day of year

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

Convert a calendar year and ordinal day number (1-366) to a YYYY-MM-DD date.

Use when you need to: iso date from day of year · date from ordinal day · nth day of year to date.

Supported

• iso date from day of year
• date from ordinal day
• nth day of year to date

Not supported

• ISO ordinal date strings
• timezones
• fiscal year

Behavior

• year is a JSON integer 1-9999. day_of_year is a JSON integer 1-366.
• day_of_year 366 is rejected unless year is a leap year.
• Output date is the zero-padded YYYY-MM-DD calendar date whose 1-based ordinal position in year equals day_of_year.

Input

• year (integer, required): min 1; max 9999
• day_of_year (integer, required): min 1; max 366

Output

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

Limits

• year min: 1
• year max: 9999

Example

Request input:

{
  "year": 2026,
  "day_of_year": 1
}

Response:

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

How to call it

MCP

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

{
  "id": "iso-date-from-day-of-year",
  "version": "1.0.0",
  "input": {
    "year": 2026,
    "day_of_year": 1
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/iso-date-from-day-of-year/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"year":2026,"day_of_year":1}'

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

CLI

node cli.mjs run iso-date-from-day-of-year 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 business day add: Add a signed count of business days to a date, skipping caller-supplied weekend weekdays and holidays.
• 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.
• ISO date range list: Enumerate the dates in an inclusive [start, end] range stepped by a whole number of days.