Julian Date Converter

Julian Date converter for astronomers, coders and historians

This Julian Date converter turns any calendar date and UTC time into a Julian Date (JD) and Modified Julian Date (MJD), and converts a JD or MJD back into a readable date. Alongside the main number it shows the day of year, the weekday, the Unix timestamp, the Julian Day Number, the days and Julian centuries elapsed since J2000.0, and the Julian and Besselian epochs, so one lookup covers everything a star catalogue, an ephemeris or a data file might ask for.

Julian Dates are the time scale astronomers use because they count days continuously, with no months, leap days or time zones to trip over. Subtracting two JDs gives an exact interval in days, which is why observations, satellite element sets and scientific databases are stamped with JD or MJD. The converter uses the algorithm from Jean Meeus’ Astronomical Algorithms, handles the switch to the Gregorian calendar on 15 October 1582, and runs entirely in your browser.

How to use the Julian Date Converter

1. Choose Date → Julian Date and enter the year, month, day and the time in UTC. Years before 1 AD use astronomical numbering (0 = 1 BC, −1 = 2 BC).
2. Read the Julian Date in the highlighted panel; MJD, day of year, weekday and Unix time appear below it with the other epochs in the table.
3. Switch to Julian Date → date and type a JD such as 2451545.0, or an MJD such as 51544.5, to get the calendar date and time.
4. Press Now (UTC) for the current Julian Date, or the J2000.0 and Unix epoch shortcuts to check reference points.
5. Use Copy JD or Copy all values to paste the results into a spreadsheet, script or logbook.

Julian Date Converter features

What a Julian Date is

The Julian Date is the number of days, including the fraction of a day, since noon Universal Time on 1 January 4713 BC in the proleptic Julian calendar. That starting point was chosen by Joseph Scaliger in 1583 as the last time three calendar cycles coincided, and it has the practical advantage that every historical observation has a positive JD. A JD ending in .5 is therefore midnight UTC, and .0 is noon, which is why astronomers’ days “start at noon” — a night’s observing never straddles a date change.

The Modified Julian Date is simply JD − 2400000.5. It starts at midnight on 17 November 1858, is small enough to fit comfortably in old record formats and is the stamp used by satellite tracking, geodesy and many data archives. The Julian Day Number (JDN) is the integer part of the JD at noon, and J2000.0 — JD 2451545.0, 1 January 2000 12:00 TT — is the reference epoch for modern star positions and the sidereal time formula.

Reference values

If a program disagrees with these rows, it is probably treating midnight as .0, using local time instead of UTC, or ignoring the 1582 calendar change.

The conversion formula

For a date Y-M-D (with January and February counted as months 13 and 14 of the previous year), Meeus’ formula is JD = ⌊365.25 (Y + 4716)⌋ + ⌊30.6001 (M + 1)⌋ + D + B − 1524.5, where B = 2 − A + ⌊A/4⌋ with A = ⌊Y/100⌋ for Gregorian dates and B = 0 for Julian-calendar dates. The time of day is added as a fraction: 18:00 UTC adds 0.75. The reverse algorithm peels those steps off again. To go from a Unix timestamp instead, divide the seconds by 86400 and add 2440587.5 — the Unix timestamp converter handles the human-readable side of that.

Where you will meet JD and MJD

• Astronomy — variable-star light curves, comet and asteroid ephemerides, exoplanet transit times, catalogue epochs and the times of lunar phases.
• Satellites — two-line element sets give their epoch as a year and fractional day; tracking software such as the ISS pass predictor works in MJD internally.
• Computing and history — databases store JD or MJD because one number sorts and subtracts cleanly, and chronologists use JDN to compare Julian, Gregorian, Islamic and Hebrew dates without ambiguity.