The Julian (old) Calendar was introduced 46 BC by Julius Caesar. It was then altered during the reign of Augustus (number of days a month). In May of 1923 the synod of Constantinople proposed the adoption of a new calendar that would drop 13 days and have a new leap year formula. So, until 2800, the Revised Julian (New Calendar) will be identical to the Gregorian. The Churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, and Bulgaria use the New Calendar. The Churches of Jerusalem, Russia, Serbia, and Georgia use the Julian or Old Calendar. Old Calendarists such the Holy Synod in Resistance use the old calendar as well, obviously. In the Churches of Poland, Albania, Czech Lands and Slovakia and the OCA, the calendar used varies from parish to parish. The autonomous Finnish and Estonian (EP) Churches use the Gregorian calendar I believe.
The synod also proposed a new Paschalion (means to determine the date of Pascha). All Churches rejected it, but the Finnish and Estonian (EP) use the Gregorian Paschalion.
I won't go into some of the arguments for and against the new/old calendars, unless that is what you wanted.