I logged that October 5-14, 1582, unaccounted-for gap as a bug against Windows. (SQL Server gets the weekday via a WinAPI call.)
I was told that this will be “soon” corrected for the cultures (languages) that were on the Roman calendar at that time. Win guys are not exactly sure what to do about countries that converted later; most notably Russian Orthodoxy, which converted only after the October Revolution of 1917. (What do you do if the date and time are marked as GMT but the culture is one of the Eastern Orthodoxies? Any suggestions for precedence of calculation?) [/quote
They are going to have real fun for Italy, which at the time was a bunch of city-states some of which converted in 1582 and some of which didn't. Also with Germany and bits of Austria (Holy Roman Empire) which converted piece-meal at later dates. And with the GB locale (different year numbers in different countries), the Netherlands, France, and Denmark. Even in the USA Texas converted to dates back when it was still Spanish, much of the rest of the USA rather later, and the Louisana Territory switched at yet another time. I guess the best they can do (unless they have the sense to do nothing) will be to cobble together some arbitrary meaningless mess which will offend various cultures quite seriously, and pretend they've done something useful.
This goes all the way back to The Bard and his oft-quoted “time is out of joint,” which alluded to the fact that when he wrote Hamlet, the English calendar was out of sync with the Roman one.
It goes back a long way before that. Some states used 1Jan as official year beginning from more than 300 years before Shakespeare, but the Roman official year never began then.
I guess this means that someone will have to think through comparisons of dates on servers that are set to different cultures. (The mentioned October Revolution, which was logged on October 25, 1917, is now celebrated on November 7.)