Julian day is the continuous count of days since the beginning of the Julian Period used primarily by astronomers. The Julian Date (JD) of any instant is the Julian day number for the preceding noon plus the fraction of the day since that instant. The term "Julian date" also refers to the day-of-year number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry.
Julian day is the continuous count of days since the beginning of the Julian Period used primarily by astronomers.
The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time, with Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar). For example, the Julian day number for January 1, 2000, was 2,451,545.
The Julian Date (JD) of any instant is the Julian day number for the preceding noon plus the fraction of the day since that instant. Julian Dates are expressed as a Julian day number with a decimal fraction added. For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is 2,456,293.520833.
The term "Julian date" may also refer, outside of astronomy, to the day-of-year number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry,— or it may refer to dates in the Julian calendar. For example, if a given "Julian date" is "May 12, 1629", this means that date in the Julian calendar (which is May 22, 1629, in Gregorian calendar— the date of the Treaty of Lübeck). Outside of an astronomical or historical context, if a given "Julian date" is "40", this most likely means the fortieth day of a given Gregorian year, namely February 9. But the potential for mistaking a "Julian date" of "40" to mean an astronomical Julian Day Number (or even to mean the year 40 ad in the Julian calendar, or even to mean a duration of 40 astronomical Julian years) is justification for preferring the terms "ordinal date" or "day-of-year" instead. In contexts where a "Julian date" means simply an ordinal date, calendars of a Gregorian year with formatting for ordinal dates are often called "Julian calendars", in spite of the potential for misinterpreting this as meaning that the calendars are of years in the Julian calendar system.
The Julian Period is a chronological interval of 7980 years beginning 4713 BC. It has been used by historians since its introduction in 1583 to convert between different calendars. 2014 is year 6727 of the current Julian Period. The next Julian Period begins in the year 3268 AD.
