Julian Date Converter
Astronomical Julian Date (and MJD) ↔ Gregorian UT.
Overview
The Julian Date Converter translates between the astronomical Julian Date (JD), the Modified Julian Date (MJD), and the Gregorian civil calendar in Universal Time. Enter either a JD/MJD number or a Gregorian timestamp and the tool returns the other representations, plus the day of the week.
Useful for astronomers logging observations, satellite operators time-stamping telemetry, archaeologists dating historical events, and anyone working with NASA, ESA, or USGS data sets that use continuous day numbering rather than month-day-year tuples.
How it works
Julian Date is a continuous count of days since noon UTC on 1 January 4713 BC in the proleptic Julian calendar (which corresponds to 24 November 4714 BC in the proleptic Gregorian calendar). It increments by exactly one each day at noon UTC. JD 0 = -4712-01-01 12:00 UT, JD 2451545.0 = 2000-01-01 12:00 UT (the J2000.0 epoch widely used in astronomy).
Modified Julian Date (MJD = JD − 2400000.5) starts at midnight UTC on 17 November 1858 and rolls over at midnight, which is more convenient for civil applications. The conversion uses the standard Meeus algorithm (chapter 7 of Astronomical Algorithms), accurate over the entire JD range and the .NET DateTime Gregorian range.
Examples
2026-05-18 12:00 UT → JD 2461179.0 MJD 60678.5
2000-01-01 12:00 UT → JD 2451545.0 MJD 51544.5 (J2000.0)
1969-07-20 20:17 UT → JD 2440423.345 (Apollo 11 lunar landing)
JD 2400000.5 → 1858-11-17 00:00 UT (MJD epoch)
FAQ
Why does the day start at noon?
Originally so a single night's astronomical observations would all share one integer date instead of spanning midnight. MJD was introduced later to start at midnight for civil convenience.
What is J2000.0?
The standard astronomical epoch — JD 2451545.0, equal to 2000-01-01 12:00 TT (Terrestrial Time). It is the reference instant for star catalogues, orbital elements, and astronomical coordinate systems.
Does it handle leap seconds?
Julian Date is normally defined relative to Universal Time (UT1 or UTC), which contains leap seconds. The difference between UT and TT (Terrestrial Time) is about 69 seconds in 2026 and matters only for sub-second astronomical work.
What is the difference between JD and MJD?
MJD = JD − 2400000.5. MJD is shorter to write, rolls over at midnight rather than noon, and is the form used by most satellite telemetry systems.
Why is the BC date 4713 or 4714?
The Julian Period was chosen so that several long astronomical and historical cycles coincide on day zero. The year reads 4713 BC under the proleptic Julian calendar (with no year zero) and 4714 BC under the proleptic Gregorian (with a year zero).