In article ,
says...
Friday, 15 October 1582 for the Gregorian Calendar!
Speaking of which... Years ago, at IBM, I saw a presentation by
a programmer there, Bruce Ohms, who devised a system for computing
the number of days between dates.
From Wikipedia:
https://en.wikipedia.org/wiki/Lilian_date
"Lilian dates can be used to calculate the number of days between any
two dates occurring since the beginning of the Gregorian calendar. It is
currently used by date conversion routines that are part of IBM Language
Environment (LE) software."
This is a pretty big deal for some computations involving financial
instruments that date back hundreds of years.