There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC.
This dating system was devised in 525 by Dionysius Exiguus of Scythia Minor, but was not widely used until after 800.
Thus, depending on the time and place, the year number changed on different days in the year, which created slightly different styles in chronology: but the date is estimated through two different approaches - one by analyzing references to known historical events mentioned in the Nativity accounts in the Gospels of Luke and Matthew, and the second by working backwards from the estimation of the start of the ministry of Jesus.
During the first six centuries of what would come to be known as the Christian era, European countries used various systems to count years.
However, BC is placed after the year number (for example: AD 2018, but 68 BC), which also preserves syntactic order.