Convert Date to Julian Day
online date to Julian Day converter. Returns JD, JDN, MJD, and J2000 offset with Julian/Gregorian switch. Client-side, instant, secure.
Compute Julian Date (JD), Julian Day Number (JDN), Modified Julian Date (MJD), and J2000 offset for any calendar date - with an auto Julian/Gregorian switch at Oct 15, 1582.
How to Use Convert Date to Julian Day
- Pick a date. The date picker defaults to today. Supports any Gregorian calendar date your browser accepts (typically back to 0001-01-01).
- Optional time - defaults to 12:00 so the JD is an integer (noon-to-noon is the astronomical convention). Any other time makes the JD fractional.
- Calendar selector. Auto uses the Julian calendar for dates before 1582-10-15 and Gregorian from that day onward (the historical switch). Override to Force Gregorian for proleptic calculations or Force Julian for consistently-old dates.
- UTC vs local. Keep the "Time is UTC" box ticked for astronomical work. Uncheck it if you're entering a local-timezone time; the tool subtracts your browser's UTC offset before computing JD.
- Press "Compute" (or just change any input - the result updates live). You'll see four values: JD (fractional), JDN (integer), MJD (= JD − 2400000.5), and days from J2000.
- Copy or download. Copy gives you a 4-value text block for pasting into lab notes or observatory logs. Download saves it as a
julian-day-*.txtfile.
Frequently Asked Questions
What’s the difference between JD, JDN, and MJD?
JD (Julian Date) is a real number: the continuous count of days (with fractions) since noon UT, January 1, 4713 BC in the proleptic Julian calendar. JDN (Julian Day Number) is the integer part – specifically floor(JD + 0.5), so it changes at midnight UT. MJD (Modified Julian Date) = JD − 2400000.5, a newer convention (IAU, 1957) that makes the numbers shorter and has MJD change at midnight.
Why does Julian Day start at noon UTC?
Because astronomers defined it that way in 1583 (Joseph Scaliger) – their working night ran across a midnight, so anchoring day boundaries at midday made bookkeeping easier. The modern astronomical day still runs noon-to-noon. Civil calendar dates change at midnight, which is why JDN and JD disagree for half of each day.
Why does the epoch go back to 4713 BC?
Scaliger picked January 1, 4713 BC because it’s the last time three astronomical cycles – the 28-year solar cycle, the 19-year Metonic cycle, and the 15-year indiction cycle – all aligned at their start. Before that date, no JD number existed; after it, every moment has a unique integer count.
What is the J2000 epoch?
J2000.0 is a standard reference instant: January 1, 2000 at 12:00 TT (≈12:00 UT) = JD 2451545.0. Orbital elements, star catalog positions, and ephemerides are typically given relative to J2000. The “J2000 offset (days)” output is how many days your input is before (negative) or after (positive) that instant.
What do I do about the Julian-to-Gregorian calendar switch?
In October 1582, Pope Gregory XIII ordered the calendar jump from Oct 4 straight to Oct 15 to correct accumulated drift. With auto mode, the tool applies the Julian rule for dates before Oct 15, 1582 and Gregorian from that day on – matching Meeus’s “Astronomical Algorithms”. Override to Force Gregorian if you want proleptic dates (valid back to antiquity under Gregorian rules).
What happens with October 5-14, 1582?
Those ten days don’t exist in the Gregorian calendar. In force Gregorian mode the tool rejects them with a specific error. In auto or force Julian mode they are valid Julian dates and compute as usual – 1582-10-14 Julian is the day before 1582-10-15 Gregorian (JD 2299160 vs 2299160.5).
Why does February 1900 have 28 days here but 1900 is divisible by 4?
Gregorian leap-year rule: divisible by 4 AND NOT divisible by 100, UNLESS divisible by 400. So 1900 and 1700 and 1800 are NOT leap years, but 2000 IS. The Julian calendar used the simpler “every 4 years is leap” rule – which is why the Gregorian reform skipped 10 days to catch up.
Are astronomical year 0 and 1 BC the same?
Astronomically, yes. Astronomical year numbering uses 0 for 1 BC, −1 for 2 BC, and so on. Historical year numbering skips from 1 BC to AD 1 (no year zero). Julian Day calculations use the astronomical convention – a year 0 entered in the date picker (if your browser allows it) is what historians call 1 BC.
How precise is the calculation?
The Meeus formula is exact to the integer day for any date from 4713 BC onward. Fractional JD from time-of-day is exact to the second (JavaScript’s double can represent ~1 µs precision within current JD ranges before losing bits). The tool rounds outputs to 6 decimals for display – sub-second.
Is the tool free, offline, and private?
Yes. Everything computes in your browser with plain JavaScript. No date is uploaded, no result is logged, no account is required. Load once and the tool works offline indefinitely.