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Julian Date Converter

Convert calendar dates to Julian Dates and back.

in-browser

How to use

  1. 1 Choose whether to convert a calendar date to a Julian Date, or the reverse.
  2. 2 Enter a UTC date and time, or a Julian Date number.
  3. 3 Read the Julian Date, Modified Julian Date and Day Number, or the calendar date.
  4. 4 Copy the result you need.

About Julian Date Converter

The Julian Date Converter translates between ordinary calendar dates and the continuous day-count that astronomers, satellite operators and historians rely on.

A Julian Date (JD) is the number of days, including the fractional part, that have elapsed since noon Universal Time on January 1, 4713 BC.

Because it is a single uninterrupted number with no months, leap years or time-zone quirks, it makes calculating the interval between two events trivial, which is exactly why it underpins ephemerides and observation logs.

Enter a calendar date and time and the tool returns the full Julian Date, the Modified Julian Date (MJD = JD − 2400000.5, which begins at midnight and keeps the numbers smaller), and the integer Julian Day Number.

Switch direction and it goes the other way, turning a Julian Date back into a precise UTC calendar instant you can copy.

Familiar reference points fall out cleanly: the Unix epoch is JD 2440587.5 and the J2000.0 standard epoch is JD 2451545.0.

Dates are interpreted in UTC to avoid time-zone ambiguity, and impossible dates such as February 30 are rejected with a clear message.

Every conversion runs locally in your browser, so nothing is uploaded and it keeps working offline.

FAQ

What is the difference between JD and MJD?

The Modified Julian Date is the Julian Date minus 2400000.5. It uses smaller numbers and starts at midnight UTC rather than noon, which is convenient for modern dates.

Which time zone does the converter use?

All dates are interpreted and displayed in UTC so that a Julian Date is unambiguous, exactly as it is defined in astronomy.

What Julian Date is the year 2000 epoch?

The J2000.0 standard epoch, 2000-01-01 at 12:00 UTC, is Julian Date 2451545.0.