🔢 Julian Day (JD) Converter

Convert a Gregorian calendar date and time (UTC) to Julian Day (JD) and Modified Julian Day (MJD), or convert a JD/MJD value back to a Gregorian date and time.

Julian Day (JD)

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What is Julian Day (JD)?

Julian Day (JD) counts every day as a single continuous number, starting from noon UTC on January 1, 4713 BCE (JD 0). Because it ignores weekdays, months, and leap-year rules, it makes calculating elapsed days, orbital periods, and calendar research much simpler — which is why it's widely used in astronomy and satellite orbit calculations. Note: JD begins at noon (12:00 UTC), not midnight.

Why do we need Modified Julian Day (MJD)?

JD values are large (over 2.4 million today) and noon-based, which can feel unintuitive for everyday use. To address this, Modified Julian Day (MJD) is defined as JD - 2400000.5. MJD starts at midnight UTC on November 17, 1858 (MJD 0), giving a shorter number that's anchored to midnight rather than noon. For example, 2000-01-01 12:00 UTC is JD 2451545.0, MJD 51544.5.

How the conversion works

Gregorian → JD uses the standard Meeus algorithm. JD → Gregorian uses Meeus's inverse algorithm; the fractional part of the entered JD/MJD is converted into hours, minutes, and seconds (UTC) for display.

Frequently asked questions

Why does JD start at noon instead of midnight?

Historically, astronomers defined the day as starting at noon so that a night of observation wouldn't straddle a date change. This convention persists today, and MJD (midnight-based) is offered alongside for convenience.

What is the relationship between JD and MJD?

MJD = JD - 2400000.5. This conversion includes an offset of exactly half a day, since JD is noon-based and MJD is midnight-based.

What is the JD value for 2000-01-01 12:00 UTC?

JD = 2451545.0, MJD = 51544.5. This is a common reference epoch in astronomical calculations (J2000.0).