Astronomy · Reference
Modified Julian Date (MJD)
A shorter, midnight-based cousin of the Julian Date used in satellite tracking, geodesy and computing. Convert to and from MJD below, then read the exact definition.
MJD ⇄ date converter
- Julian Date (JD)
- Modified JD (MJD)
- Julian Day Number
Tip: set the scale selector to MJD to enter a Modified Julian Date directly.
Definition
The Modified Julian Date is defined by a single, exact relationship:
MJD = JD − 2400000.5
The .5 is the important part: the Julian Date begins each day
at noon, so subtracting half a day makes the Modified
Julian Date begin at midnight, matching civil dates. The
epoch MJD 0 is 00:00 UTC on 17 November 1858.
Quick facts
- MJD = JD − 2,400,000.5
- JD = MJD + 2,400,000.5
- Epoch: 1858-11-17 00:00 UTC
- Increments at midnight UTC
Where MJD is used
- Satellite & spacecraft tracking — compact time tags that fit legacy fixed-width formats.
- Geodesy and Earth-orientation data — the IERS publishes parameters against MJD.
- Radio and X-ray astronomy — pulsar timing and light curves are routinely plotted on an MJD axis.
- Computing — a small integer day count that is easy to store and difference.
Related
Frequently asked questions
What is the Modified Julian Date (MJD)?
The Modified Julian Date is the astronomical Julian Date minus 2,400,000.5. Subtracting the 2.4 million removes redundant leading digits, and the extra 0.5 shifts the start of the day from noon to midnight, so the MJD increments at 00:00 UTC like a civil date.
Why was the MJD created?
It was introduced by the Smithsonian Astrophysical Observatory in 1957 for satellite tracking. The full Julian Date needs many digits and rolls over at noon; the MJD is shorter, fits in fewer bits, and changes at midnight, which is far more convenient for computing and geodesy.
What is the MJD today?
Today the MJD is in the 60,000s. Because MJD = JD − 2,400,000.5, you can get it from any Julian Date by subtracting that constant. The converter above shows the exact value for any date.
When does the MJD epoch start?
MJD 0.0 corresponds to 00:00 UTC on 17 November 1858 (JD 2,400,000.5).