But the difference between the 2 calendars changes by 3 days every 400 years.
Julius Caesar established a leap year every 4 years.
This led to the departure of the equinox from the start of spring over the centuries.
Pope Gregory adjusted the calendar by 10 or so days (to fix the error) and removed leap years from 3 days in every 400 years (no longer do we have leap days on century years that are not divisible by 4: no leap year on 1700 1800 1900 but is one on 2000).
Since the Orthodox do not follow the Pope, they did not make the change.
In the US we switched
As British Colonies Sept 3 1752 became Sept 9 1752
In Alaska October 6 1867 became October 18 1867
Russia did not switch 'til 1918 and Romania 'til 1919
China started using the Gregorian Calendar in 1912, but never used the Julian Calendar
For other countries see
polysyllabic.com/?q=book/export/html/25