What is a Julian Day Number?

The Julian Day Number (JDN) is an integer count of days since noon on January 1, 4713 BC (Julian calendar), also written as JD 0.0. It is the basis of the Julian Date (JD) system, which adds a fractional part for the time of day. Noon UT on any date is always an exact half-integer JD.

What is the difference between JDN and JD?

JDN is the integer part — the day count at noon UT. JD (Julian Date) includes a fractional part for the time: JD 0.0 = noon Jan 1, 4713 BC, so JD 0.5 = midnight starting Jan 2, 4713 BC. Most ephemeris work uses the full decimal JD.

What is the Modified Julian Date (MJD)?

MJD = JD − 2 400 000.5. This shifts the epoch to midnight November 17, 1858, and produces smaller numbers that fit in 32-bit integers. MJD is standard in satellite geodesy and radio astronomy.

When should I use the Julian calendar option?

Use the Julian calendar for dates before October 15, 1582, the date of the Gregorian reform. Historical records from Ancient Rome, medieval Europe, and early modern times use the Julian calendar. After October 4, 1582 (Julian) the Gregorian calendar took over in Catholic countries.

Why does the Julian Date start at noon, not midnight?

The convention dates to 19th-century astronomers who observed through the night. Starting the JD at noon meant the date did not change during a single night's observing session, making logs easier to keep.

How accurate is this tool?

The tool uses the standard Meeus / IAU algorithm valid for any proleptic Gregorian or Julian date. Accuracy is exact for integer JDNs and accurate to within seconds for fractional JDs limited by JavaScript floating-point precision.

Julian Day Number Converter

Name: Julian Day Number Converter
Availability: InStock
Author: Nham Vu

Convert between calendar dates (Gregorian or Julian) and Julian Day Numbers used in astronomy and scientific time calculations.

Year

Month

Day

Hour (UT)

Minute

Second

Calendar System

Gregorian Julian

Enter a date or Julian Day Number and click Convert.

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Summary

Convert between calendar dates (Gregorian or Julian) and Julian Day Numbers used in astronomy and scientific time calculations.

How it works

Select the conversion direction: calendar date to JD, or JD to calendar date.
For date-to-JD: enter the year, month, day, and optional time (UT), then choose Gregorian or Julian calendar.
For JD-to-date: enter a Julian Day Number (decimal values supported for fractional days).
The tool applies the standard IAU algorithm to compute the exact Julian Day Number and Julian Date.
Results are shown instantly with the JDN, fractional JD, Modified Julian Date (MJD), and the equivalent calendar date.

Use cases

Convert observation timestamps to Julian Dates for astronomy logs.
Calculate the number of days between two historical dates.
Look up the calendar date for a Julian Day Number from an ephemeris table.
Convert between Old Style (Julian calendar) and New Style (Gregorian calendar) dates.
Compute Modified Julian Date (MJD) for satellite tracking or radio astronomy.
Verify dates in ancient history that span the Julian/Gregorian calendar reform of 1582.
Use JD arithmetic to add or subtract days from a calendar date.
Cross-check date conversions for software or scientific data pipelines.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu