Convert Julian Day to Date

All three are continuous day-count systems. JDN (Julian Day Number) starts at noon UTC on 1 January 4713 BC and is the astronomers’ original – its day boundary at noon is awkward for civil use. MJD (Modified Julian Day) = JDN − 2,400,000.5; the half-day subtraction moves the day boundary to midnight UTC and the smaller number is friendlier for code. TJD (Truncated Julian Day, NASA) = JDN − 2,440,000.5; even smaller, useful for spacecraft logs in the modern era.

A JDN with a fractional part encodes time-of-day. 2451545.0 is exactly noon UTC on 1 January 2000. 2451545.25 is 6 hours later = 18:00 UTC. 2451545.5 is midnight UTC on 2 January 2000. Many older tools silently truncate the fraction – this one does not, so 18:00 UTC actually appears in the result.

J2000.0 is the standard reference epoch for star catalogs, planetary ephemerides, and orbital element sets. It’s defined as JDN 2,451,545.0, which is exactly 12:00 TT (Terrestrial Time) on 1 January 2000 – and very close to 12:00 UTC. Try entering it: you’ll see Saturday, 2000-01-01 12:00:00 UTC.

From JDN modulo 7. JDN 0 was a Monday. Each successive JDN advances one weekday, wrapping at 7. The current tool computes the day-of-week from the floor of JDN+0.5 (the civil day for the UTC midnight that started) so it always matches the displayed Gregorian date.

For dates after 1582 (Catholic Europe) or 1752 (British) the Gregorian one is what people use. For dates before that, historical sources often use the Julian Calendar – and the two diverge by up to 14 days for the far past. Both are computed from the same JDN so you can cross-check against primary sources.

Only for dates within the ISO 8601 range (year 1 through 9999). Earlier dates (like JDN 0 = 4713 BC) show “out of Unix range” because the Unix timestamp formula and JS Date both reject negative-year datetimes. The JDN/MJD/TJD/Gregorian fields still populate correctly outside that range.

The Fliegel & Van Flandern algorithm (Comm. ACM 1968) is exact for the entire Gregorian and Julian calendar ranges from 4713 BC to 9999 AD. Differences between this output and NASA HORIZONS or U.S. Naval Observatory tables are due to time-system conventions (TT vs UT1 vs UTC), not the date arithmetic.

Seven columns: input, jdn, mjd, gregorian (with UTC time), julianCalendar, dayOfWeek, unix. Header row included. Paste directly into Excel or Google Sheets – tabs split into cells automatically. Invalid rows show “ERR” in the second column so you can spot them.

No. All arithmetic runs in your browser using integer math – no network calls, no analytics. Open DevTools → Network and confirm zero requests fire after the page loads.

Yes. Total bundle is under 22 KB. Once loaded you can disconnect and keep converting. Useful for fieldwork or archival research where you’re computing from paper observation logs without internet.