Catholic countries adopted the modern Gregorian calendar in 1582, while Britain and her colonies observed the Julian calendar until 1752. By default, Mathematica uses the British calendar, so dates in Catholic countries after October 14, 1582, and before September 14, 1752, require the Gregorian option.
In France, 1700 was not a leap year:
Britain was still using the Julian calendar where all centuries were leap years:
Catholic countries did not delete any days in September 1752:
The Russian Orthodox church has retained the Julian calendar. Their year 1900 was a leap year:
The Islamic calendar has leap years (the 12 month has 29 or 30 days):