Who uses a pre-"Gregorian" calendar and why?

Question

Here's a "day job" question that apparently has its roots in history.

A person with one of our (overseas) suppliers declined to work this week, because it's his Christmas holiday. He's Orthodox, and is using an "old" calendar some days behind the current (Gregorian) one.

My understanding was that the current calendar came into being, when Pope Gregory decreed that too many days (relative to e.g., the winter solstice) had passed since the birth of Christ. To compensate, he "jumped" the calendar ten days forward in the 16th century, and thereafter, there would be no leap year in three "centennial" years out of four. (There WAS a leap year in 2000, but not in 1900 or 2100).

Apparently, the Protestant countries took over a century (until the early 18th century), before making the switch. More to the point are still a few holdouts, even today.

Which group of Orthodox Christians (or others) are still on the old calendar? Why did they fail to switch to the new one, (unlike the Protestants)?

Answer 1

No major Orthodox Church follows the Gregorian calendar. Most follow the Milanković calendar (the "new calendar"), and the Russian, Serbian and Georgian Orthodox Churches, and the Greek Orthodox Church of Jerusalem follow the Julian calendar (the "old calendar"). The monasteries in Mount Athos follow the Julian calendar for religious issues, and the Milanković calendar for civil issues (as they are technically part of Greece).

The Greek Orthodox Patriarchate of Constantinople was the first to adopt the Milanković calendar in 1924, and the Church of Greece followed shortly after. In the following years, all other major Orthodox Churches - except the four mentioned in the first paragraph - adopted the "new calendar".

This lead to a schism in Eastern Orthodoxy, with several "old calendar" Churches breaking communion with the "new calendar" Churches. "Old calendarists" maintain that there's no theological reason for adopting the "Pope's calendar", and some (e.g. the Genuine Orthodox Christians) believe the switch constitutes heresy.