So how would the people who pressed the coin know it was 250BC?
In 250BC the birth of Christ was a future event whose date was unknown.
Assuming they could have known (worked it out from the Hebrew scriptures, perhaps, as the Three Wise Men might have done), how then would they have known that the calendar would be numbered from Christ's birth year?
Assuming they could have known that, how would they have known we'd get it wrong and Christ was actually born around 6BC, thus making 250BC actually 244BTABOC? (before the actual...)

The only possible explanation is that a coin stamped 250BC is a fake.