A Julian Day (JD) is a continuous count of days since noon Universal Time on 1 January 4713 BCE in the proleptic Julian calendar. The system was proposed by Joseph Scaliger in 1583 and is now standard in astronomy.
How it works
Each day increments the Julian Day number by 1. The day begins at noon, not midnight, so a single astronomical observation night carries one JD value. Today, 28 May 2026, is approximately JD 2,461,189. The United States Naval Observatory publishes the formula for converting between Gregorian dates and Julian Day numbers.
Where it is used
Julian Days simplify 2 common technical problems:
- Astronomy — observation timestamps, orbital calculations, and ephemeris tables.
- Software — internal date storage and elapsed-day arithmetic.
Subtracting 2 Julian Day numbers gives the exact number of days between 2 dates, with no calendar conversion needed. This avoids edge cases around leap years and month boundaries.
Distinct from "Julian date"
The term "Julian date" is often used informally to mean an ordinal date, such as day 150 of 2026. This usage is common in supply chain and military contexts. It is not the same as a Julian Day number. Astronomers reserve "Julian Day" for the continuous count described here.