Date Converters
Julian Day Converter
Convert a calendar date to its Julian Day Number and back — the continuous day count used in astronomy and software.
Gregorian → Julian Day
Julian Day Number
2,461,205.5
- JD at noon
- 2,461,206
- Day of week
- Sunday
Julian Day → Gregorian
Enter a Julian Day Number.
Where Julian Days are used
Because the Julian Day is just a number, it is the simplest way to find the exact number of days between two dates, even across centuries or different calendar systems. It is widely used in astronomy, satellite tracking, chronology and databases, where storing a single day count is easier than handling months and leap years.
Frequently asked questions
- What is a Julian Day Number?
- The Julian Day Number (JDN) is a continuous count of days since the beginning of the Julian Period — noon on 1 January 4713 BC. Astronomers and software use it because it turns any date into a single number, making it easy to calculate the number of days between events across different calendars.
- Is the Julian Day the same as the Julian calendar?
- No. Despite the similar name, the Julian Day is a running day count used in astronomy, while the Julian calendar is the older calendar introduced by Julius Caesar. This tool converts between the modern Gregorian calendar and the Julian Day Number.
- Why does the Julian Day change at noon?
- Julian Days traditionally start at noon Universal Time so that a single night of astronomical observation falls on one Julian Day. We show both the day-number form and the value at noon for convenience.