Julian Date Converter
Free, with no sign up. Calculations run on your device; your entries stay in this browser.
Enter a timestamp like 2000-01-01T12:00:00Z (optional 1–3 millisecond digits), or select the reverse direction and enter a decimal Julian day. Years 0001–9999 use the proleptic Gregorian calendar throughout, including before 1582: no historical calendar switch. Julian days count continuously from noon UT1 on January 1, 4713 BC in the Julian calendar. Here each civil UTC day is modeled as 86,400 seconds, so UTC-based results are a civil approximation to UT1, without Earth-orientation corrections or leap seconds. Reverse timestamps round to the nearest millisecond; the day number is floating-point and displayed to 14 significant digits. This is not a precision astronomical time-scale converter.
Your result
JD = 2440587.5 + civil milliseconds since 1970-01-01T00:00:00Z ÷ 86,400,000. Reverse: civil milliseconds = round((JD − 2440587.5) × 86,400,000). Midnight has a .5 day fraction; noon has an integer day number. Ordinal day is the separate 1–365/366 day-of-year count.
Primary source: US Naval Observatory, Julian date definition and Gregorian conversion; USNO, UT1 time scale and noon epoch.
Worked example
1970-01-01T00:00:00Z gives JD 2440587.5. The civil timestamp 2000-01-01T12:00:00Z gives JD 2451545.0; midnight that date gives 2451544.5. Day-of-year is 1 in all these examples.
How to use this tool
Choose the conversion direction and enter a UTC timestamp ending Z or a decimal Julian day. Select Calculate.
Frequently asked questions
Is a Julian date the same as day of year?
No. Julian day is a continuous count with a noon epoch. Ordinal day-of-year restarts at 1 each January and is shown separately.
Does this account for UT1 and leap seconds?
No. Entered UTC civil timestamps use equal 86,400-second days. Results are explicitly a civil approximation to UT1, with leap seconds and Earth-orientation corrections outside scope.
Is this free and private?
Yes. These calculations run in your browser, without sign-up or uploading your entries.