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Julian Day Number

Continuous day count used by astronomers to compare dates.

Formula

JDN=⌊365.25(Y+4716)⌋+⌊30.6001(M+1)⌋+D+B−1524JDN = \left\lfloor 365.25(Y+4716) \right\rfloor + \left\lfloor 30.6001(M+1) \right\rfloor + D + B - 1524

Variables

YYear
MMonth (Jan/Feb → 13/14 of prior year)
DDay
BGregorian correction

Example

2000-01-01 12:00 UT ⇒ JDN 2,451,545

Did You Know?

Astronomers start each Julian Day at noon so a single night's observations never straddle a date change.

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