On this page is a Julian Day calculator. Quickly figure out today's Julian Day count - or the count of any other day you can imagine.
What is Julian Day?
The Julian Date (JD) is a continuous count of days and fractions since noon on January 1, 4713 BCE (Julian calendar). The Julian Day Number (JDN) is the integer day count. Astronomers use JD/JDN to simplify calculations across long time spans without worrying about calendar reforms (or month lengths).
The Julian period was introduced by Joseph Justus Scaliger in 1583; he chose the 4713 BCE epoch because the 28‑year solar cycle, 19‑year lunar (Metonic) cycle, and 15‑year indiction cycle all align. (See more at Encyclopaedia Britannica.)
JD/JDN is widely used in ephemeris calculations, satellite tracking, and historical astronomy research. The IAU maintains working groups on Julian Date standards.
Features
- Date to JDN - Convert any Gregorian date to its Julian Day Number
- JDN to date - Convert a Julian Day Number back to a calendar date
- High precision - Uses the standard astronomical algorithm
Related calculators
- Day Number Calculator - Days since Year 1 CE
- J2000 Day Calculator - Modern astronomical epoch
- Unix Day Calculator - Computing epoch
- Sun Position Calculator - Uses Julian Day internally
