Who set AD and for what reason (date)?

When was AD introduced as a dating system alongside BC and why. Secondly what did it replace?
in interest when and how was it a introduced change and when and how was it implemented?
and if anybody does know how did the proposed birth of Christ come around i have heard mention of a 14-16th century British priest who went backwards to try and chronological date the bible from its ages (age of adam + noah ect)

Dionysius Exiguus, a sixth-century monk, was the inventor of AD or “Anno Domini” meaning “the year of Our Lord”. He was born in what is now Romania. Using the Roman system of chronology (ab urbe conditum, “from the founding of the city of Rome”), he dated the year of Christ’s birth to 754 years after Rome was founded. He also calculated the date of Easter, with the help of the nineteen-year Alexandrian cycle. By creating an entirely new dating system, he uprooted the earlier Diocletian Era (named after a third-century Roman Emperor). He wanted to stop the church from using a system named after a former oppressor of Christianity. Unfortunately, he made a small miscalculation, and it seems that Christ was actually born four years earlier than Dionysius believed.

Solomon’s Temple – Prt2(The Calendar)