todayjuliandate
Converter

About

About & Methodology

todayjuliandate.com is a focused reference for Julian dates in every sense of the term. Here's how it works and how we keep it accurate.

What this site covers

“Julian date” is used for two unrelated things — the day-of-year code used in manufacturing and IT, and the continuous day count used in astronomy. Most Julian date tools quietly pick one and ignore the other. We treat both as first-class, explain which is which, and let you convert either.

How the converters work

Every calculation runs locally in your browser — no date you enter is ever sent to a server. The conversions are powered by a single, tested JavaScript engine shared across the whole site, so the number on the home page, in the converters and in the calendars always agree.

  • Day-of-year values come from straightforward calendar arithmetic with correct leap-year handling (divisible by 4, except centuries not divisible by 400).
  • Astronomical Julian Date values use the algorithms from Jean Meeus, Astronomical Algorithms (2nd edition, 1998), chapter 7. Historical auto mode jumps from 4 October to 15 October 1582; the omitted labels are rejected instead of mapped to duplicate instants.

Accuracy & testing

The engine is covered by an automated test suite that checks it against published reference values, including:

Reference momentExpected JD
J2000.0 (2000-01-01 12:00 TT)2451545.0
Unix epoch (1970-01-01 00:00)2440587.5
MJD epoch (1858-11-17 00:00)2400000.5
Julian Period start (−4712-01-01 12:00)0.0

The reverse conversion is verified by round-tripping a dense grid of instants across 150 years and confirming they return to within a second.

Conventions we use

  • Interactive astronomical values use civil UTC on the JavaScript/Unix time model. Leap seconds are not represented. Precision work must distinguish UT1, UTC, TT and TDB.
  • Dates on or after 15 October 1582 are interpreted in the Gregorian calendar; earlier dates in the Julian calendar, keeping the day count continuous.
  • Two-digit years in day-of-year codes are resolved with a 1969–2068 window (00–68 → 2000s, 69–99 → 1900s). Use the 7-digit form to avoid ambiguity.

Found a problem?

Accuracy matters to us. If you spot a value that looks wrong or a case we don't handle, email [email protected] with the input, expected result and source. In the meantime, cross-check any mission-critical value against a second authority such as the U.S. Naval Observatory.

Primary references

Start here