Julian Date Converter
Astronomical date conversion
Julian Date Converter
Use this Julian Date Converter to convert Gregorian calendar dates into Julian Day Number and decimal Julian Date values, or convert JDN and JD values back into Gregorian dates.
Live output
Result
Pick a mode, enter a date or Julian value, and the hero turns into the result surface.
Julian Date Converter guide
Convert Gregorian dates, Julian Day Numbers, and Julian Dates
A Julian Date Converter translates a civil calendar date into the continuous day count used by astronomers and many historical date references. Instead of naming a month and day, the Julian Day system counts days from a fixed epoch, which makes it easier to compare dates across long time spans.
This homepage focuses on astronomical Julian Date conversion. It is different from a Julian calendar converter: the Julian calendar is a historical calendar system, while Julian Day Number and Julian Date are numerical date counts. Use this tool when you need JDN, JD, or a Gregorian date recovered from a Julian value.
For Gregorian input, the converter returns both the integer Julian Day Number and the decimal Julian Date at midnight UTC. For Julian input, it accepts either an integer JDN or a decimal JD and normalizes the value back into a Gregorian calendar date.
What this converter handles
Julian Date conversion options
Gregorian to JDN/JD
Enter a Gregorian calendar date and return the integer Julian Day Number plus the decimal Julian Date at 00:00 UTC.
Julian to Gregorian
Paste a Julian Day Number or decimal Julian Date and convert it back into a Gregorian calendar date.
Astronomy reference dates
Work with continuous day counts used in astronomy, chronology, ephemerides, archives, and date normalization.
How to use the converter
How to convert a Julian Date
Choose the direction that matches your source value, enter the date or number, and run the conversion. The result area shows the normalized output and the related reference value.
Choose Gregorian to JDN/JD
Use this direction when you have a standard Gregorian date such as 2026-06-30 and need the Julian Day Number or decimal Julian Date.
Choose Julian to Gregorian
Use this direction when your source is a Julian Day Number such as 2461222 or a decimal Julian Date such as 2461221.5.
Check the UTC convention
The displayed decimal Julian Date is calculated for 00:00 UTC. Because astronomical Julian days begin at noon, midnight values commonly end in .5.
Reference examples
Julian Date Converter examples
These examples show the kind of values the converter is designed to handle. Results can be used as reference points when checking astronomy notes, date-indexed datasets, or historical timelines.
Gregorian date to Julian values
2026-06-30 -> JDN 2461222, JD 2461221.5A Gregorian date entered at midnight UTC returns an integer day number and a decimal Julian Date with the midnight offset.
Julian Date to Gregorian date
2461221.5 -> 2026-06-30A decimal Julian Date can be converted back into the corresponding Gregorian calendar date.
Julian Day Number to Gregorian date
2461222 -> 2026-06-30An integer JDN is normalized through the Julian day that starts at noon UTC, then displayed as the matching Gregorian calendar date.
FAQ
Julian Date Converter FAQ
What is a Julian Date Converter?
A Julian Date Converter changes a Gregorian calendar date into a Julian Day Number or decimal Julian Date, and can also convert a JDN or JD value back into a Gregorian date.
What is the difference between JDN and JD?
JDN is the integer Julian Day Number. JD is a decimal Julian Date that can include the time of day. At 00:00 UTC, a JD often ends in .5 because Julian days begin at noon UTC.
Is Julian Date the same as the Julian calendar?
No. Julian Date and Julian Day Number are numerical day counts. The Julian calendar is a historical calendar with months and leap-year rules. Use the Julian Calendar Converter page for historical Julian calendar dates.
What input formats are accepted?
Gregorian dates use YYYY-MM-DD format. Julian input can be an integer Julian Day Number or a decimal Julian Date.
Why does the decimal Julian Date end in .5?
Astronomical Julian days start at noon UTC. A Gregorian date at midnight UTC is therefore half a day into the Julian day count, which is why the decimal value commonly ends in .5.
Reference
Julian Date reference source
The Julian Date convention used here follows the astronomical practice of counting days from a fixed epoch, with Julian days beginning at noon Universal Time.
US Naval Observatory Julian Date notes
The US Naval Observatory describes Julian Date as a continuous count of days and fractions from the beginning of the year -4712, with the Julian day beginning at noon UT.
Read the USNO Julian Date reference