julian number calendar
The Julian Day number is the number of days elapsed since noon (12:00) on January 1, 4713 BC The Modified Julian Day number is the number of days since midnight on November 17, 1858 AD (MJD=JD-2400000.5) The Gregorian calendar is used for dates on and after October 15, 1582 AD and the Julian calendar is used before October 4, 1582.