Greek Old Calendarism

Greek Old Calendarism is an Eastern Orthodox heresy, whose followers use the Julian calendar - also called the "Old Style Calendar", "Church Calendar" or "Old Calendar" - and are s not in communion with the Eastern Orthodox churches that use the Revised Julian calendar which aligns with the Gregorian calendar.

Archiopedists would write more about the subject, but they think that Wikipedians have already said it best.