Anno Domini
The system for working out the years was invented by Dionysius Exiguus in about AD 525. He fixed the point Anno Domini, which is used to number the years of both the Gregorian calendar and the Julian calendar. He used it to identify the several Easters in his Easter table. He did not use it to date any historical event. When he devised his table, Julian calendar years were identified by naming the consuls who held office that year — he himself stated that the "present year" was "the consulship of Probus Junior [Flavius Probus]", which he also stated was 525 years "since the incarnation [conception] of our Lord Jesus Christ". How he arrived at that number is unknown. He invented a new system of numbering years to replace the Diocletian years that had been used in an old Easter table because he did not wish to continue the memory of a tyrant who persecuted Christians.