Original sentence: Both the Anno Domini system and the more recent Common Era system of reckoning years have their origin in a faulty 6th Century attempt to calculate the year of his birth, which most now estimate would have been from 8 BC/BCE to 4 BC/BCE
Edited sentence: Both the Anno Domini system and the more recent Common Era system of reckoning years have their origin in a faulty 6th Century attempt to calculate the year of his birth, which most now estimate would have been from 8 BCE to 4 BCE (or 8 BC to 4 BC)