AD (abbreviation for Anno Domini, which translates to the year of our lord)/BC was made by Dionysius Exiguus in 525 to update the Easter table from the Diocletian era calendar due to Exiguus’ wish to not memorialize Diocletian who led the last large persecution of Christians. There are two major theories on how Exiguus devised the years one being the use of the gospel of Luke that states that Jesus was thirty during the 15th year of Tiberius Caesar. It did take awhile to become widespread, almost 300 years thanks to the Carolingian Renaissance and took some countries even longer such as Portugal which adopted it in 1422

Blackburn, Bonnie; Holford-Strevens, Leofranc (2003). The Oxford companion to the Year: An exploration of calendar customs and time-reckoning. Oxford University Press

Declercq, Georges (2000). Anno Domini: The origins of the Christian era. Turnhout: Brepols

Declercq, G. "Dionysius Exiguus and the Introduction of the Christian Era". Sacris Erudiri 41 (2002):