onlinecalculator.me

Julian date converter

The Julian Day Number is a continuous count of days since January 1, 4713 BCE used in astronomy to compare dates across calendar systems. Enter a Gregorian date or a JDN to.

About this calculator

Formula

Gregorian date → JDN (Meeus, “Astronomical Algorithms” Ch. 7)

a = floor((14 − month) / 12)
y = year + 4800 − a
m = month + 12·a − 3
JDN = day + floor((153·m + 2) / 5) + 365·y + floor(y/4) − floor(y/100) + floor(y/400) − 32045

Adding a time component

JD = JDN + (hour − 12)/24 + minute/1440 + second/86400

At noon UTC the fractional part is 0, so JD = JDN exactly.

Derived quantities

QuantityFormula
Modified Julian Date (MJD)JD − 2400000.5
J2000 offset (days)JD − 2451545.0

Worked example

Date: January 1, 2000 at 12:00:00 UTC (J2000 epoch)

  • JDN = 2451545
  • JD = 2451545.0 (noon)
  • MJD = 51544.5
  • J2000 = 0.0

Known reference points

DateJDN
Jan 1, 1970 (Unix epoch)2440588
Jan 1, 2000 (J2000 epoch)2451545
Jan 1, 20242460311

Notes

  • The algorithm handles proleptic Gregorian dates (dates before the Gregorian calendar was adopted).
  • Dates before October 15, 1582 were in the Julian calendar era; this converter uses the proleptic Gregorian formula for all dates.

Frequently asked

What is a Julian Day Number?
A Julian Day Number (JDN) is an integer count of days since the start of the Julian Period — January 1, 4713 BCE in the Julian calendar, noon UTC. JDN 0 is that starting day. January 1, 2000 is JDN 2451545. The system gives astronomers a single timeline that spans any historical date.
What is a Julian Date vs. a Julian Day Number?
A Julian Day Number is an integer (the whole-day count). A Julian Date (JD) adds a fractional part representing the time of day, anchored to noon UTC. At noon on January 1, 2000, JD = 2451545.0 exactly.
What is the Modified Julian Date?
The Modified Julian Date (MJD) = JD − 2400000.5. It resets the origin to midnight on November 17, 1858, producing smaller numbers convenient for modern calculations. MJD 0 = JD 2400000.5.
What is J2000?
J2000 is the standard astronomical epoch: January 1.5, 2000 UTC (noon on January 1, 2000) = JD 2451545.0. The J2000 offset shows how many days before or after that epoch a date falls.
Is this the same as the Julian calendar?
No. The Julian Day Number is a counting system invented by astronomers. The Julian calendar is a civil calendar that predates the Gregorian calendar. The names happen to share origin (both named after Julius Caesar), but the systems are different.

Embed this calculator

Add this free calculator to your own site. Copy the snippet — it works anywhere you can paste HTML, and stays in sync with this page.

Preview embed →

Browse all calculators → · More in date time →