# 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:

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

Response:

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

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

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

### HTTP (no key)

```sh
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](/v1/tools/iso-date-from-day-of-year/versions/1.0.0).

### CLI

```sh
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/](/clients/cli/).

## Related tools

- [ISO date day of year](/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](/tools/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](/tools/iso-date-month-end): Return the last valid calendar day of the year-month of a YYYY-MM-DD date.
- [ISO date month start](/tools/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](/tools/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](/tools/iso-date-range-list): Enumerate the dates in an inclusive [start, end] range stepped by a whole number of days.