Most of the world's non-Muslim countries use the Gregorian calendar,
introduced by Pope Gregory XIII in 1582 as a reform of the Julian calendar.
(The latter, instituted by Julius Caesar, had been in use since 46 B.C.) Americans have used the Gregorian calendar
since 1752.
What is the
difference?
The principal difference between the two calendars is that
in the Gregorian system a century year must be divisible by 400 in order to
qualify as a leap year (e.g., A.D. 2000 is a leap year but not A.D. 1900). For
each century that is not divisible by 400, the Julian calendar falls one day
behind the Gregorian calendar. Thus, by 1542,
October 4 on the Julian calendar was equivalent to October 15 on the Gregorian calendar.
(The Book of Answers, by Barbara Berliner)