Byzantine calendar
The Great Schism, dated to 1054, created two religions from what was formerly one. The Western part later became the Roman Catholic Church. The Eastern part is known as the Eastern Orthodox Church. At that time the Roman Catholic church used the Julian calendar. In 1453 the Byzantine Empire collapsed ending the use of their calendar. But others still used it, notably Russia. Numbering differences began to appear when the Gregorian calendar was introduced in 1582 by Pope Gregory XIII.