Do you remember the history of how the world moved from the Julian calendar to the modern Gregorian calendar? On February 24, 1582, Pope Gregory XIII, assisted by the Italian astronomer Luigi Lilio Ghiraldi and the German Jesuit mathematician Christopher Clavius, issued a papal bull. This official letter in the Catholic Church declared, among other details, that Thursday, October 4, 1582, would be followed by Friday, October 15, 1582. The intervening 10 days would never exist. This took effect in the Papal States and Iberia, then a little later in France. However, it took nearly 200 years for England and its American colonies to adopt the change: There, September 14, 1752, followed September 2, 1752. (By this time, the error had grown from 10 days to 11 days.) Not by accident, then, do datetime values in SQL Server 2000 and 2005 start on January 1, 1753.
The adoption of the Gregorian calendar was staggered in some other parts of the world. In some cases the adoption didn't happen until relatively recently. It was adopted by Protestant Germany in 1699, by most of Scandinavia by 1700, by Japan in 1873, by China in 1911, by Russia in 1918, by Greece in 1923, and by Turkey in 1927. So remember that if you deal with historical dates, the specific date strictly depends on geographic location: The same day may have calendar dates 10 or more days apart in different countries.