What is a Julian Day Number (JDN)?

A Julian Day Number is a continuous count of days starting from noon on January 1, 4713 BC (Julian calendar). It is widely used in astronomy because it avoids the complexity of months, years, and calendar reforms.

What is the difference between the Julian and Gregorian calendars?

The Julian calendar was introduced by Julius Caesar in 45 BC and assumes every fourth year is a leap year. The Gregorian calendar, introduced in 1582, refines this by skipping leap years on century years not divisible by 400. Today most of the world uses the Gregorian calendar.

What is today's Julian Day Number?

The current JDN changes at noon UTC each day. You can use this tool with today's date to find out. For reference, January 1, 2000 at noon UTC corresponded to JDN 2,451,545.

What is the Julian Period?

The Julian Period is a 7,980-year cycle starting in 4713 BC, calculated by combining three historical calendar cycles. It was proposed by Joseph Scaliger in 1583 and forms the basis of the Julian Day Number system.

Can this tool handle dates before the common era (BC)?

Yes. Enter a negative year value for BC dates (e.g., -44 for 44 BC). Note that year 0 is used in astronomical year numbering (it corresponds to 1 BC).

What is the Julian Date (JD) vs Julian Day Number (JDN)?

Julian Date (JD) includes a fractional part representing the time of day (e.g., 2451545.0 = noon UTC on Jan 1, 2000). Julian Day Number (JDN) is the integer part only, referring to a full calendar day.

Julian Date Converter

Convert any calendar date (Gregorian or Julian) to its Julian Day Number and back — instantly.

Conversion Input

Calendar System

Year

Month

Day

Quick examples:

Result

Enter a date or JDN and click Convert

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Summary

Julian Date Numbers (JDN) are a continuous count of days since January 1, 4713 BC in the proleptic Julian calendar, widely used in astronomy and historical research. This tool converts any Gregorian or Julian calendar date to its corresponding JDN and vice versa, making it easy to compare dates across calendar systems.

How it works

Choose a conversion direction: date to JDN, or JDN to date.
For date-to-JDN, select your calendar system (Gregorian or Julian), then enter the year, month, and day.
For JDN-to-date, enter the Julian Day Number.
Click Convert to see the result in all supported formats simultaneously.
Use the swap button to reverse the conversion direction.

Use cases

Astronomy and astrophysics calculations requiring a continuous day count.
Historical research comparing dates across Gregorian and Julian calendars.
Software development involving date arithmetic across calendar systems.
Genealogy research converting old Julian calendar records to modern dates.
Academic work involving ancient or medieval date references.
Verifying date conversions in spreadsheets or databases.

Frequently Asked Questions

Last updated: 2026-05-28 · Reviewed by Nham Vu