This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
The Lilian day number is similar to the Julian day number, except that
Lilian day number 1 started at midnight on the first day of the
Gregorian calendar, that is, 15 October 1582.
The Lilian day number is named after Aloysius Lilius mentioned in