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JD and Gregorian Date Converter Convert between Julian Date numbers and Gregorian calendar dates.

Julian Date Converter illustration
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Julian Date Converter

Convert between Julian Date numbers and Gregorian calendar dates.

1

Choose Direction

Select Date → Julian or Julian → Date conversion.

2

Enter Value

Enter a date or Julian Day number.

3

View Results

See Julian Date, Modified Julian Date, day of year, and more.

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What Is Julian Date Converter?

The Julian Date Converter provides bidirectional conversion between Julian Day Numbers (JD) and Gregorian calendar dates. The Julian Day Number is a continuous count of days since January 1, 4713 BCE (Julian calendar), widely used in astronomy, geology, and other sciences for precise day counting across millennia. This tool converts any calendar date to its Julian Date (with decimal for time of day), Modified Julian Date (JD - 2,400,000.5), and Julian day-of-year format. In reverse, enter any Julian Day Number to see the corresponding Gregorian date, time, and ISO format. A "Use Current JD" button provides instant access to the present Julian Date.

Why Use Julian Date Converter?

  • Bidirectional conversion between JD and Gregorian dates
  • Includes Modified Julian Date (MJD) for modern applications
  • Julian day-of-year format (YYYYDDD)
  • Decimal precision for time-of-day representation
  • One-click copy for each result format
  • Essential for astronomical and scientific calculations

Common Use Cases

Astronomy

Calculate Julian Dates for astronomical observations, ephemerides, and orbital calculations.

Satellite Tracking

Use Modified Julian Date for satellite orbit calculations and TLE data.

Geology

Reference geological events using the continuous Julian Day count.

Historical Research

Convert ancient dates to a common reference system for cross-calendar comparison.

Scientific Computing

Use JD for day-difference calculations that span centuries or millennia.

Technical Guide

The Gregorian-to-JD conversion uses the standard algorithm: adjust months January and February to be months 13 and 14 of the previous year, then apply: JD = day + floor(153*adjustedMonth + 2)/5 + 365*adjustedYear + floor(adjustedYear/4) - floor(adjustedYear/100) + floor(adjustedYear/400) - 32045. Time of day is added as a decimal fraction. The reverse conversion (JD to Gregorian) uses the algorithm from Meeus' "Astronomical Algorithms": compute intermediate values z, a, b, c, d, e, then derive day, month, and year. Modified Julian Date is simply JD - 2400000.5, introduced by the Smithsonian Astrophysical Observatory in 1957 to avoid the large numbers in standard JD.

Tips & Best Practices

  • 1
    Julian Dates include a decimal part for time: .0 = noon UT, .5 = midnight UT
  • 2
    Modified Julian Date (MJD) is preferred in modern applications as the numbers are smaller
  • 3
    JD counts from noon (not midnight), so December 31.5 = January 1 midnight
  • 4
    The Julian Day Number is NOT the same as the Julian calendar — they are different concepts
  • 5
    Use JD for day-difference calculations across historical eras (it handles calendar changes)

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Frequently Asked Questions

Q What is the Julian Day Number?
The Julian Day Number (JD) is a continuous count of days since noon Universal Time on January 1, 4713 BCE. It provides a single number for any date in history.
Q Is Julian Date the same as the Julian calendar?
No, they are different concepts. The Julian Date is a continuous day count used in astronomy. The Julian calendar is the calendar system introduced by Julius Caesar.
Q Why does JD start at noon?
Historically, astronomers changed the date at noon because their observations happened at night — a single night would fall within one Julian Day.
Q What is Modified Julian Date?
MJD = JD - 2,400,000.5. It starts at midnight instead of noon and uses smaller numbers.
Q How precise is the conversion?
The conversion is accurate to the nearest minute for dates within the range of JavaScript's Date object (approximately 271,821 BCE to 275,760 CE).

About This Tool

Julian Date Converter is a free online tool by FreeToolkit.ai. All processing happens directly in your browser — your data never leaves your device. No registration or installation required.