Endpoints

Endpoint	Returns	Try it
/api/today.json	Today's Julian date in every format, in UTC.	run
/api/convert.json?date=YYYY-MM-DD	A calendar date → day-of-year code, JD, MJD and JDN.	run
/api/convert.json?julian=YYYYDDD	A day-of-year code → calendar date. Accepts DDD, YYDDD or YYYYDDD.	run
/api/convert.json?jd=NUMBER	An astronomical Julian Date → UTC calendar date and time.	run
/api/convert.json?mjd=NUMBER	A Modified Julian Date → UTC calendar date and time.	run
/api/calendar/YYYY.json	Every date in a year with its day-of-year code. One request.	run

Quick start

curl https://todayjuliandate.com/api/today.json

Returns:

{
  "generatedAt": "2026-09-17T09:55:49Z",
  "timezone": "UTC",
  "date": "2026-09-17",
  "longDate": "Thursday, September 17, 2026",
  "ordinal": {
    "dayOfYear": 260,
    "ddd": "260",
    "yyddd": "26260",
    "yyyyddd": "2026260"
  },
  "astronomical": {
    "jd": 2461300.91376,
    "mjd": 61300.41376,
    "jdn": 2461301
  },
  "year_info": {
    "year": 2026,
    "isLeapYear": false,
    "daysInYear": 365,
    "dayOfYear": 260,
    "daysRemaining": 105
  }
}

JavaScript

const res = await fetch('https://todayjuliandate.com/api/today.json');
const { ordinal } = await res.json();
console.log(ordinal.yyyyddd);  // "2026260"

Python

import urllib.request, json

url = "https://todayjuliandate.com/api/convert.json?julian=2026260"
with urllib.request.urlopen(url) as r:
    data = json.load(r)

print(data["date"])       # 2026-09-17
print(data["longDate"])   # Thursday, September 17, 2026

Notes

Time zone. Every response is UTC. Near midnight your local day-of-year may differ by one — if that matters, send an explicit date= rather than relying on today.json.
Caching. Conversions are pure functions of their query string and are served with a long Cache-Control. today.json is cached for 60 seconds.
Errors. Bad input returns HTTP 400 with an error message and, where useful, a hint.
Accuracy. Astronomical values follow Meeus, Astronomical Algorithms (2nd ed.), ch. 7, and switch to the Julian calendar before the 1582 reform. See Julian Day for the background.
Fair use. It is free and unmetered. If you are going to hammer it, cache the response — a day-of-year code changes once a day.

One line, no dependencies, no network request of its own — it computes the date from the visitor's own clock, so it is always right and always instant.

<script src="https://todayjuliandate.com/widget.js"></script>

With options:

<script src="https://todayjuliandate.com/widget.js"
        data-format="full"
        data-theme="auto"></script>

Attribute	Default	Values
data-format	yyyyddd	yyyyddd · yyddd · ddd · jd · mjd · full
data-theme	auto	auto · light · dark
data-tz	local	local · utc
data-link	true	Set to false to drop the credit link
data-target	—	CSS selector to render into, instead of in place

As an iframe

If a script tag is not allowed where you need it, embed the standalone page instead:

<iframe src="https://todayjuliandate.com/embed/"
        width="320" height="150" frameborder="0"
        title="Today's Julian date"></iframe>

Licence

Both the API and the widget are free for any use, including commercial. No attribution is required, though a link back to todayjuliandate.com is appreciated and is what keeps them free.

Julian Date API & Widget