The Gregorian Calendar
Notes from this site
An interesting historical sidelight on the Gregorian Calendar is that not all countries adopted it immediately.
In particular, it was adopted uniformly in Catholic countries, but Protestant countries often still used the Julian Calendar.
Thus, the date could change by 10 days simply by crossing certain country borders!
England and its American colonies did not adopt the Gregorian Calendar until 1752,
when 11 days were removed from the calendar, and Russia resisted
this change until after the 1917 Revolution.
One conseqence of the British adoption of the Gregorian Calendar in 1752 is that
George Washington was born on February 11, 1731,
according to the calendar in use on his birthday,
but we now celebrate his date of birth as February 22, 1731
(actually, even that is no longer true with the advent of Presidents Day)
For more information on this subject read this interesting web site page:-