jdn

JDN stands for Julian Day Number. It is the integer that represents a day in the Julian day count, a continuous time scale used primarily in astronomy. The count starts at noon UTC on January 1, 4713 BCE (in the Julian calendar), with that moment designated as JDN 0. Each subsequent noon increments the JDN by one, so the day is the interval between successive noons rather than midnight to midnight.

JDNs are computed by converting calendar dates to the continuous day count. For dates in the Gregorian calendar, a common algorithm uses integer arithmetic to produce JDN from year, month, and day, handling the calendar reform that shifted many dates in October 1582. A widely used form is: a = (14 - month) / 12; y = year + 4800 - a; m = month + 12*a - 3; JDN = day + (153*m + 2)/5 + 365*y + y/4 - y/100 + y/400 - 32045. Dates in the Julian calendar use a related, simpler calculation. Conversions are implemented in many astronomical libraries and calculators.

Applications of JDN include ephemerides, celestial navigation, spacecraft trajectory planning, and historical chronology. Because JDNs are simple integers and do not depend on time zones, they provide a stable, unambiguous basis for comparing dates across long time spans and different calendars.

A commonly cited reference point is JDN 2451545, which corresponds to 2000-01-01 at 12:00 TT. Related concepts include the Modified Julian Date (MJD) and standard time scales such as Universal Time and Terrestrial Time, which are used in conjunction with Julian day calculations.