Calendars and time
Modified Julian Date
Direct answer
Modified Julian Date: Modified Julian Date, or MJD, equals Julian Date minus 2,400,000.5, placing its day boundary at midnight rather than noon.
MJD is a shortened midnight-based form of Julian Date.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Unix Time, Reduced Julian Date, Calendar, Julian Date
MJD is a shortened midnight-based form of Julian Date.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Modified Julian Date
MJD was introduced in the nineteenth century and has an epoch at 1858 November 17 00:00.
Why Modified Julian Date matters
Modified Julian Date reduces large day numbers and aligns integer changes with civil midnight for many scientific datasets.
Where readings of Modified Julian Date go wrong
The usual misreading is to drop the 0.5 in the conversion or omit the time scale at high precision.
Sources and editorial method
Research references used for this entry include:
- NASA Goddard: Modified Julian Dates - NASA explanation of JD and MJD conventions and their reference epochs.
- U.S. Naval Observatory: Julian Date Converter - Official explanation of Julian dates and conversion between calendar dates and continuous day counts.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Convert JD and MJD correctly
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Common questions
What is Modified Julian Date?
Modified Julian Date, or MJD, equals Julian Date minus 2,400,000.5, placing its day boundary at midnight rather than noon.
What do people get wrong about Modified Julian Date?
The usual misreading is to drop the 0.5 in the conversion or omit the time scale at high precision.
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