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 ) .
The Julian day or Julian day number ( JDN ) is the number of days that have passed since the initial epoch defined as noon Universal Time ( UT ) Monday , January 1 , 4713 BC in the Julian calendar .