Calendars and time

Julian Date

Direct answer

Julian Date is a continuous count of days and fractions of a day from a defined epoch, widely used to represent astronomical time without month and year boundaries.

Calendars & TimekeepingThe Great Almanac DictionaryUpdated 2026-09-26

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At a glance

Found in: Calendars & Timekeeping
Related: Calendar, Solar Calendar, Lunar Calendar, Lunisolar Calendar

An engraved clock movement with an hourglass and a sundial — Calendar and clock · visual guide **Julian Date**
Julian Date is a continuous astronomical count of days and day fractions.
A cycle or timeline schematic for comparing the named timekeeping concept.

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The story behind Julian Date

The system uses an epoch at Greenwich noon on 1 January 4713 BCE in the proleptic Julian calendar and was designed for chronological computation.

Why Julian Date matters

A continuous count simplifies time intervals, software calculations, and comparisons across civil calendar changes.

Julian Date in practice: an example

Astronomical software can subtract two Julian dates directly to find the elapsed number of days.

Where readings of Julian Date go wrong

A common error is to call a Julian calendar date a Julian Date, and do not omit the time scale when precision matters.

Sources and editorial method

Research references used for this entry include:

U.S. Naval Observatory: Julian Date Converter - Official explanation and conversion service for Julian dates and civil calendars.
U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical coordinates, events, calendars, and time scales.

Source: The Great Almanac Dictionary
Category: Calendars & Timekeeping
Last updated: 2026-09-26

Convert a civil date to Julian Date

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CalendarA Calendar organizes days into named and numbered cycles for reckoning time. Solar CalendarA Solar Calendar keeps the calendar year aligned with the seasons. Lunar CalendarA Lunar Calendar bases its months on recurring Moon-phase cycles. Lunisolar CalendarA Lunisolar Calendar coordinates lunar months with the solar seasonal year. Coordinated Universal TimeCoordinated Universal Time is the international reference standard for world time.

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