How Did Man Know BC From AD?

I understand how we count years since our Lord’s death- EG, 1248ad, 1249ad, 1250ad, etc.
How did we get the the “count-down” years before Christ was born? EG, 1250bc, 1249bc, 1248bc, etc? What was the starting point and how is it man knew to count downward?

Q. I understand how we count years since our Lord’s death- EG, 1248ad, 1249ad, 1250ad, etc.
How did we get the the “count-down” years before Christ was born? EG, 1250bc, 1249bc, 1248bc, etc? How was it that the ancients “knew” to count time downward to the promised, prophesied event of the coming Messiah?

I guess my question is, what was the starting point and how is it man knew to count downward? Very perplexing!

A. Understanding our calendar and its origins is a complicated matter and I won’t attempt to explain it all here. Simply speaking, the BC (before Christ) dates were all developed after the fact, and are counted backward from the traditional time of the Lord’s birth (which was established by the Roman Emperor Domitian and is off by at least 4 and perhaps as many as 7 years).

For the most part only nations that in the past described themselves as Christian measure time this way. For example, this year is 5768 on the Jewish calendar, indicating their belief that it’s been 5768 years since the creation. And in Islam this year is 1429, since their calendar began in 622 AD and is a lunar calendar having only 354 days.

According to Daniel 7:25, the anti-Christ will try to change the world’s calendars again, presumably to erase any reference to the Lord.