Julian Date Converter

Use this Julian date converter to switch between a Gregorian calendar date and astronomical Julian Date formats. In Gregorian to Julian mode, enter a date and Time UTC in HH:mm format. In Julian to Gregorian mode, enter a numeric Julian Date. The calculator returns JD, MJD, RJD, TJD, or the matching Gregorian UTC date.

How to use this converter

Select the conversion direction first. For a calendar date, use the date picker and enter time in 24-hour UTC, such as 12:00. For a Julian Date, enter the full decimal value, because the fractional part represents time of day. If you are comparing local timestamps, convert the local time with the time zone calculator first.

How it works

For Gregorian dates, the calculator applies the standard astronomical date algorithm, including the Gregorian calendar correction after October 15, 1582.

$JD = integer\ calendar\ terms + day + time\ fraction - 1524.5$

It then reports common offsets:

$MJD = JD - 2400000.5$

$RJD = JD - 2400000$

$TJD = JD - 2440000.5$

Worked example

A Gregorian date of 2023-02-25 at 00:00 UTC converts to a Julian Date around 2460000.5. Entering 2460000.5 in reverse mode returns the corresponding UTC calendar timestamp.

Avoiding convention mix-ups

Astronomers use continuous day counts; manufacturers often use day-of-year codes. For ordinary elapsed time, use the time duration calculator, and for age or anniversaries use the age calculator.