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’s the difference between the the Julian or the Gregorian calendar?
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.