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Julian Day (JD) ○◂｜Definition｜1st｜20251119205401-00-⌔
Julian day
The Julian day is a continuous count of days from the beginning of the Julian period; it is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g., food production date and expiration date).1
The Julian period is a chronological interval of 7980 years, derived from three multi-year cycles: the indiction, solar, and lunar cycles. The last year that was simultaneously the beginning of all three cycles was 4713 BC (−4712),2 so that is year 1 of the current Julian period, making AD 2026 year 6739 of that Period. The next Julian period begins in the year AD 3268. Historians used the period to identify Julian calendar years within which an event occurred when no such year was given in the historical record, or when the year given by previous historians was incorrect.3
The Julian day number (JDN) has the same epoch as the Julian period, but counts the number of days since the epoch rather than the number of years since then. Specifically, Julian day number 0 is assigned to the day starting at noon Universal Time on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar).4567 For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2 451 545.8
The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. Julian dates are expressed as a Julian day number with a decimal fraction added.9 For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2 456 293.520 833.10 This article was loaded at 2026-06-27 18:47:11 (UTC) – expressed as a Julian date this is 2461219.2827662.
Printed 2026-06-28.
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Link to original Footnotes
“Julian date” n.d. ↩
Astronomical Almanac for the year 2017 p. B4, which states 2017 is year 6730 of the Julian period. ↩
Grafton 1975 ↩
Dershowitz & Reingold 2008, 15. ↩
Seidelman 2013, 15. ↩
“Astronomical Almanac Online” 2016, Glossary, s.v. Julian date. Various timescales may be used with Julian date, such as Terrestrial Time (TT) or Universal Time (UT); in precise work the timescale should be specified. ↩
Both of these dates are years of the Anno Domini or Common Era (which has no year 0 between 1 BC and AD 1). Astronomical calculations generally include a year 0, so these dates should be adjusted accordingly (i.e. the year 4713 BC becomes astronomical year number −4712, etc.). In this article, dates before October 15, 1582, are in the (possibly proleptic) Julian calendar and dates on or after October 15, 1582, are in the Gregorian calendar, unless otherwise labelled. ↩
McCarthy & Guinot 2013, 91–92 ↩
“Resolution B1” 1997. ↩
US Naval Observatory 2005 ↩
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