Julian day

Julian day is used in the Julian date (JD) system of time measurement for scientific use by the astronomy community, presenting the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon. Julian date is recommended for astronomical use by the International Astronomical Union.
The Julian Day Number (JDN) is the Julian day with the fractional part ignored. It is sometimes used in calendrical calculation, in which case, JDN 0 is used for the date equivalent to Monday January 1, 4713 BC in the Julian calendar.
The term Julian date is widely used to refer to the day-of-year (ordinal date) although incorrectly.