"AD (or A.D.) is an abbreviation for the Latin expression "Anno Domini", which translates to "the Year of Our Lord", and equivalent to C.E. (the Common Era). Anno Domini refers to the years which followed the supposed birth year of the philosopher and founder of Christianity, Jesus Christ. For the purposes of proper grammar, the format is properly with the A.D. before the number of the year, so A.D. 2018 means "The Year of Our Lord 2018", although it is sometimes placed before the year as well, paralleling the use of B.C.
The choice of starting a calendar with the birth year of Christ was first suggested by a few Christian bishops including Clemens of Alexandria in C.E. 190 and Bishop Eusebius at Antioch, C.E. 314–325. These men labored to discover what year Christ would have been born by using available chronologies, astronomical calculations, and astrological speculation."
https://www.thoughtco.com/christian-church-history-underlies-calendars-169928