Simplify the following text for a Grade 11 native English-speaking person.
Original: Dionysius is best-known as the inventor of the Anno Domini era , which is used to number the years of both the Gregorian calendar and the ( Christianized ) Julian calendar .
Simplified: Dionysius is best-known as the inventor of the Anno Domini era , which is used to number the years of both the Gregorian calendar and the Julian calendar .