Astronomy · Converter
Julian Day Converter
Convert between a UTC date and time and the astronomical Julian Date (JD), Modified Julian Date (MJD) and Julian Day Number (JDN) — precise to the second.
Convert JD / MJD ⇄ date
- Julian Date (JD)
- Modified JD (MJD)
- Julian Day Number
Julian Date, precisely defined
The Julian Date is the interval of time in days and fractions of a day since 12:00 UT on 1 January 4713 BC in the proleptic Julian calendar (Greenwich noon, Monday). It gives astronomers a single real number for any instant, so the interval between two events is just a subtraction — no months, weeks or leap years to reconcile.
The Julian day-count family
| Scale | Definition | Epoch |
|---|---|---|
| Julian Date (JD) | — | 4713 BC Jan 1, 12:00 UT |
| Modified JD (MJD) | JD − 2400000.5 | 1858 Nov 17, 00:00 |
| Reduced JD | JD − 2400000 | 1858 Nov 16, 12:00 |
| Truncated JD (TJD) | ⌊JD − 2440000.5⌋ | 1968 May 24, 00:00 |
| Dublin JD (DJD) | JD − 2415020 | 1899 Dec 31, 12:00 |
| CNES JD | JD − 2433282.5 | 1950 Jan 1, 00:00 |
Accuracy
Conversions use the algorithms in Jean Meeus, Astronomical Algorithms (2nd ed., chapter 7), verified against reference epochs including J2000.0 (JD 2451545.0) and the Modified Julian Date epoch. The tool automatically applies the Julian calendar before the 1582 Gregorian reform. All values are in Universal Time (UTC); it does not apply the small ΔT correction to Terrestrial Time.
Related
Frequently asked questions
What is the Julian Day right now?
The astronomical Julian Date is a continuous count of days since noon Universal Time on 1 January 4713 BC. Today it sits in the 2,461,000s. Because the count ticks over at noon UTC, the value "now" usually has a fractional part rather than a whole number.
What is the difference between Julian Date and Julian Day Number?
The Julian Day Number (JDN) is the integer day count assigned to a whole day beginning at noon UTC. The Julian Date (JD) is the JDN plus the fraction of the day elapsed since that noon, so it carries the time of day.
Why does the Julian Day start at noon?
The convention dates to astronomers who observed at night and did not want the date to change in the middle of an observing session. Starting each day at noon keeps a whole night within a single Julian Day.
Does this converter handle dates before 1582?
Yes. It follows the astronomical convention: dates on or after 15 October 1582 use the Gregorian calendar, and earlier dates use the Julian calendar, so the day count stays continuous across the reform.