Calendars and time
Reduced Julian Date
Direct answer
Reduced Julian Date is a shortened Julian-date convention often defined as JD minus 2,400,000, though usage has not always been uniform.
Reduced Julian Date is a historical shortened JD convention whose offset must be stated.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Modified Julian Date, Julian Century, Calendar, Julian Date
Reduced Julian Date is a historical shortened JD convention whose offset must be stated.
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The story behind Reduced Julian Date
Several reduced day-count notations were created to save printed columns and machine storage before standards converged.
Why Reduced Julian Date matters
Reduced Julian Date’s ambiguity makes source metadata essential when digitizing older astronomical and geophysical records.
Reduced Julian Date in practice: an example
A historical table labeled RJD may differ by 0.5 day from an MJD table even when their printed numbers look similar.
Where readings of Reduced Julian Date go wrong
A common error is to infer the definition from the abbreviation alone; verify the publication convention.
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
Verify the reduced-date offset
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More in Calendars & Timekeeping
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Common questions
What is Reduced Julian Date?
Reduced Julian Date is a shortened Julian-date convention often defined as JD minus 2,400,000, though usage has not always been uniform.
What do people get wrong about Reduced Julian Date?
A common error is to infer the definition from the abbreviation alone; verify the publication convention.
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