Julian Day Number Converter

Summary

The Julian Day Number (JDN) is a continuous count of days since the beginning of the Julian Period on January 1, 4713 BC. It is the standard time scale used in astronomy, ephemeris calculations, and scientific databases to avoid calendar ambiguity. This tool converts any Gregorian or Julian calendar date to a Julian Day Number and Julian Date (JD), and also reverses the conversion from a JD back to a calendar date.

How it works

1. Select the conversion direction: calendar date to JD, or JD to calendar date.
2. For date-to-JD: enter the year, month, day, and optional time (UT), then choose Gregorian or Julian calendar.
3. For JD-to-date: enter a Julian Day Number (decimal values supported for fractional days).
4. The tool applies the standard IAU algorithm to compute the exact Julian Day Number and Julian Date.
5. 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