Simplify the following text for a Grade 9 native English-speaking person.
Original: This calendar replaced the Ecclesiastical Calendar based on the Julian Calendar in use by the Eastern Orthodox Church since the first Ecumenical Council of Nicea in 325 AD . The Revised Julian Calendar aligned its dates with the Gregorian Calendar proclaimed in 1582 by Pope Gregory XIII for adoption by the Roman Catholic Church .
Simplified: The Revised Julian calendar is a variation of the Julian calendar .