Let’s say, Jesus was born on December 25th as it is he common opinion among religious persons. In our calendar, the year when Jesus was born is considered to be the year before Christ: 1 BC. The year after Jesus was born is considered to be the year AD 1. So there is no year 0 in the Anno Domini system. The year 1 BC ended on December 3st1 and the year 1 AD started at January the first. So, the Anno Domini system tells us, that the days from December 26th till December 31st in the year were Jesus was born are “before Christ”, even if Christ was already born on December 25th. Or in other words: Christ was born before Christ.
I know, this is just a theoretical question and when Jesus was born, other calendars were used and it is not even sure, when exactly Jesus was born. But anyway, it’s amusing that the Anno Domini system refer to the birth of Christ as a starting point but doesn’t take into consideration that there are 6 days between his birth and the end of the year “before Christ”.