Check out this previous answer by /u/Algernon_Asimov:
At what point did society begin referring to the year in modern terms (2012 etc)
There's always room for further answers, but this answer by /u/sunagainstgold on a slightly different question does give a good history of the use of the current year dating system: How confident are we that the year is actually and exactly 2016? Is it possible that at some point in the last 2000 years there were any significant timekeeping mistakes?