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Julian Date Converter: Calendar Date to JD and Back

This free tool converts a Gregorian calendar date and time (UTC) into the Julian Date (JD) used in astronomy, and the Modified Julian Date (MJD). It also turns a Julian Date back into a calendar date.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

Astronomers count days instead of using calendar dates. The Julian Date makes it easy to find the gap between two events, even hundreds of years apart.

Enter a date and time in UTC, or a Julian Date.

How to Use This Tool

  1. Enter or paste your input in the box or boxes above.
  2. Choose the options, if there are any.
  3. Press the button to run the tool.
  4. Read the result and use the copy button where one is shown.

How the Calculation Works

The tool counts the days from 1970-01-01 to your date, adds the time as a fraction of a day, and adds 2440587.5, which is the Julian Date of the Unix epoch. For the reverse direction it subtracts 2440587.5 and turns the days back into a date and time. MJD is JD minus 2400000.5.

Worked Example

1 January 2000 12:00 UTC is JD 2451545.0. 1 January 2024 00:00 UTC is JD 2460310.5 and MJD 60310.

Common Uses

  • Astronomy and astrophotography.
  • Satellite and science data.
  • Long-span date maths.
  • Software that uses JD.

Tips and Common Mistakes

  • Times are in UTC, not local time.
  • The date picker shows only years supported by your browser.
  • JD of 0 is in 4713 BC and is not supported.
  • Check the first result against a case you already know, so you can trust the tool for the rest.
  • Nothing you type leaves your device. The work is done by your browser, so it also works with a slow connection once the page has loaded.

Next, you may need the Unix timestamp converter, week number calculator, day of the week calculator, date difference calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

A continuous count of days since noon UTC on 1 January 4713 BC. Astronomers use it to avoid calendar problems.
No. The Julian Date is a day count. The Julian calendar is an old calendar. This tool uses the modern Gregorian calendar for dates.
MJD = JD - 2400000.5. It starts at midnight and has fewer digits, so satellites and data systems use it.
Because it was set so that an astronomer's night falls inside one day number. A Julian day starts at 12:00 UTC.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.