Two men A and B found a coin on road on one side of the coin was a king and the other side it was written 200 BC A says it to be a fake coin B says on the other way who is right why?

Two men A and B found a coin on road on one side of the coin was a king and the other side it was written 200 BC A says it to be a fake coin B says on the other way who is right why?
No coin can hold a date containing BC(Before Christ), since 200 years bc no one knew that Christ would be born 200 years after.
if the coin had said 200Ad it might have been genuine