The Julian date (JD) is a timekeeping system used by astronomers to avoid the ambiguities and computational complexities of the civilian calendar; the 'Julius' involved is not Julius Caesar, and this system is unrelated to the Julian calendar. The Julian date is the number of days that have elapsed since noon (12h Universal Time) on Jan 1, 4713 BC. For example, January 1, 1970 is JD 2440588. Decimal fractions correspond to fractions of a day so that, for example, an observation made at 15h on Jun 24, 1962 is given as JD 2437840.63; the whole number part is called the Julian date number.
The system was proposed by the French scholar Joseph Justus Scaliger (1540–1609) in 1582 and named after his father, Julius Caesar Scaliger. His choice of starting year was based on the convergence in 4713 BC of three calendrical cycles, one of which was the 15-year ancient Roman tax cycle of Emperor Constantine, so is of no practical consequence. For convenience, the modified Julian date (MJD) is sometimes used. This is defined as starting at midnight on November 17.0, 1858, so that MJD = JD - 2,400,000.5 day.