Simplify the following text for a Grade 9 native English-speaking person.
Original: Although all Eastern Orthodox countries ( most of them in Eastern or Southeastern Europe ) had adopted the Gregorian calendar ( or in the case of Greece , possibly the Revised Julian calendar ) by 1927 , their national churches had not .
Simplified: Some countries used this calendar to the 20th century . Mount Athos , and many national Eastern Orthodox churches still use the Julian calendar , or a revised form , introduced in 1923 .