Julian Date Calculator
No months, no leap years, no calendar reforms.
Work out Julian Date. No months, no leap years, no calendar reforms. Free, with no account and nothing to install.
Julian day
2,451,545
1 January 2000, a Saturday · MJD 51,544.5
Julian day counts continuously from noon on 1 January 4713 BC, with no months, no leap years and no calendar reforms in the arithmetic. That is exactly why astronomy uses it — the interval between two dates becomes plain subtraction. The day begins at noon rather than midnight, which is why the fractional part is 0.5 at midnight. Astronomers chose noon so that a single night's observations all fall on the same Julian day. Modified Julian Day drops the leading 24 and shifts the start to midnight, which is more convenient for anything that has to line up with civil dates. Both are in current use, and mixing them is a half-day error.
How the Julian Date Calculator works
Julian Day and Modified Julian Day from a calendar date, with day of week, day of year, days from J2000.0 and the Unix epoch, and Greenwich mean sidereal time.
Also known as: convert date to julian day · modified julian date converter · days since j2000 · what day of the week was that
Frequently asked questions
What is a Julian day?
A continuous count of days from noon on 1 January 4713 BC. It has no months, leap years or calendar reforms in the arithmetic, so the interval between two dates is plain subtraction.
Why does the Julian day start at noon?
So that a single night's astronomical observations all fall on the same Julian day. It is why the fractional part is 0.5 at midnight.
What is Modified Julian Day?
Julian Day minus 2,400,000.5 — dropping the leading digits and shifting the start to midnight. More convenient for anything aligning with civil dates, and mixing the two is a half-day error.
What is J2000.0?
The epoch at JD 2451545.0 — noon UTC on 1 January 2000 — used as the reference for modern star catalogues and orbital elements.
Does it handle the Gregorian reform?
Yes. Dates from 15 October 1582 use the Gregorian correction and earlier ones use the Julian calendar, which is why the ten days that never happened do not break the count.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/julian-date-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Julian Date Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/julian-date-calculator" style="color:#64748b">Julian Date Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.