Julian Day Number Converter
Convert between calendar dates (Gregorian or Julian) and Julian Day Numbers used in astronomy and scientific time calculations.
Decimal values supported. E.g. 2451545.0 = J2000.0
Enter a date or Julian Day Number and click Convert.
Julian Day Number
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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