Julian Date
A continuous count of days since noon Universal Time on January 1, 4713 BCE.
The Julian Date (JD) is a continuous count of days and fractions of days since noon Universal Time on January 1, 4713 BCE. It was introduced by astronomers to provide a single system of dates for working with different calendars.
The Julian Date system was proposed by Joseph Scaliger in 1583. The starting point was chosen because it is before recorded history and represents the point at which three major cycles used in ancient chronology coincided.
Julian Dates are expressed as decimal numbers, where the integer part represents the day number and the fractional part represents the time of day.
A variant called the Modified Julian Date (MJD) is defined as MJD = JD - 2400000.5, which reduces digits and places the integer boundary at midnight.
Julian Dates are primarily used in astronomy and space science to simplify time interval calculations.