This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
Sometimes a modified Julian day number (MJD) is used which is
2,400,000.5 less than the Julian day number. This brings the numbers
into a more manageable numeric range and makes the day numbers change
at midnight UTC rather than noon.
MJD 0 thus started on 17 Nov 1858 (Gregorian) at 00:00:00 UTC.