hi! you may be interested in this post from the FAQ section How did the world agree on what year it is? - check out the FAQ for more on how the Gregorian calendar was developed

• How certain are we that the year 1 AD was 2012 years ago?

other related posts

• How sure are we that the year is actually 2014?

• How certain are we of what year it is? Were there every any disagreements, like during the Dark Ages or afterwards, of the exact year?

• How do we know this is really 2014?

• How do we know what year it is?

tangentially-related

• What is the earliest known event to which we know the exact year it occurred?

• What is the earliest recorded date that we can determine accurately?