Julian Day Converter (JD, MJD, JDN)

Convert a calendar date and UTC time into the astronomical Julian Date (JD) — the continuous day count used in astronomy — and convert a Julian Date back into a calendar instant. The tool also shows the Modified Julian Date (MJD), the integer Julian Day Number, and days since the J2000.0 epoch. Everything runs live in your browser.

How to use the Julian Day Converter (JD, MJD, JDN)

Set a date and time in the calendar field and the Julian Date updates instantly, or type a Julian Date such as 2451545.0 and the calendar instant fills in. The time is treated as UTC, which is the convention for Julian Dates in astronomy. Because a Julian Date rolls over at noon rather than midnight, midnight UTC always lands on a .5 fraction — 1970-01-01 00:00:00 is JD 2440587.5.

The cards show four related values: the full Julian Date with its fractional day, the Modified Julian Date (MJD = JD − 2400000.5, which starts each day at midnight and is common in satellite and geodesy work), the integer Julian Day Number for the day containing that instant, and the number of days since the J2000.0 epoch (noon UTC on 1 January 2000). The standard check value is built in: noon UTC on 1 January 2000 is exactly JD 2451545.0.

It all runs in your browser, so conversions are instant, work offline, and nothing you enter is uploaded anywhere.

What a Julian Date is and why astronomers use it

A Julian Date is a single continuously increasing number that counts days — and fractions of a day — from a fixed origin far in the past. The count begins at noon Universal Time on 1 January 4713 BC in the proleptic Julian calendar, a deliberately ancient zero point chosen so that every date in recorded history has a positive Julian Date. The idea, introduced by Joseph Scaliger in 1583 and later adopted by astronomers, is to strip away all the irregularities of civil calendars — leap years, varying month lengths, calendar reforms, time zones — and replace them with one clean real number. The difference between two events is then just a subtraction, with no calendar arithmetic at all, which is exactly what you want when computing how long ago a star was observed or how many days until a planetary alignment.

The choice of noon as the day boundary is not arbitrary: it means a single night of observing never spans a date change, since the rollover happens in the middle of the day when astronomers are asleep. That convenience for astronomers is mildly inconvenient for everyone else, because civil midnight then falls on a half-day. To smooth this over, the Modified Julian Date was defined as MJD = JD − 2400000.5; subtracting the .5 shifts the rollover back to midnight, and subtracting the large constant gives a smaller, more manageable number (MJD passed 60000 in 2023). MJD is the form used in satellite tracking, GPS, and geodesy. A third common reference, J2000.0, is the instant JD 2451545.0 — noon UTC on 1 January 2000 — which serves as the modern epoch for star catalogues and orbital elements, so many formulas are expressed in days or centuries since J2000.

Converting between a Julian Date and a civil calendar date is a compact piece of arithmetic. Going forward, you compute the integer Julian Day Number for the date with a well-known formula and then add the fractional day from the time of observation, remembering to offset by twelve hours because the count starts at noon. Going backward, you split the Julian Date into its integer and fractional parts and run the inverse formula to recover the year, month, and day, then turn the fraction back into hours, minutes, and seconds. The formulas handle the Gregorian calendar correctly across leap years and century boundaries, which is why a tested converter is handy whenever you are working with observation logs, ephemeris files, two-line element sets, or any scientific data that timestamps events in Julian or Modified Julian Dates.

Common use cases

• Astronomy. Turn an observation timestamp into a Julian Date for ephemeris and orbit calculations.
• Satellite tracking. Convert to and from Modified Julian Date, the form used in TLEs and geodesy.
• Day counting. Find the exact number of days between two events with a single subtraction.
• Epoch math. Compute days since J2000.0 for formulas expressed relative to that epoch.