Julian Day Converter
Convert any date to Julian Day Number, Modified Julian Date and Unix epoch days.
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The Julian Day Number is a continuous count of days used in astronomy, space science, and historical chronology. It eliminates the complexity of calendar systems - no months, no leap rules, just a simple day count. Astronomers use it for calculating intervals between celestial events, programmers reference the Unix epoch (JDN 2440588) and satellite operators use Modified Julian Date for telemetry.
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Frequently asked questions
The Julian Day Number (JDN) is a continuous count of days since January 1, 4713 BC (Julian calendar). January 1, 2025 has JDN 2,460,677. It provides a universal timeline independent of calendar systems.
MJD = JDN - 2,400,000.5. It was introduced to reduce the large JDN numbers and to start the day at midnight rather than noon. MJD 0 corresponds to November 17, 1858. Widely used in satellite operations.
The Unix epoch (January 1, 1970 00:00 UTC) has JDN 2,440,588. To convert JDN to Unix days: subtract 2,440,588. To get Unix seconds, multiply by 86,400.
No. The Julian Day Number is a day count invented by Joseph Scaliger in 1583. The Julian Calendar is the pre-Gregorian calendar introduced by Julius Caesar in 46 BC. They share a name but are different concepts.
J2000.0 is the standard astronomical epoch: January 1, 2000 at 12:00 TT (JDN 2,451,545). Satellite ephemerides, star catalogs and planetary positions are referenced to this epoch.