If atheists were asked to define their own calendar era, (in the sense that it is now 2014 AD, 1435 AH etc.) what event would they choose as the starting point?

Use the https://en.wikipedia.org/wiki/Holocene_calendar*. Just add the digit 1 in front of any Common Era date, adding 10,000. This year is 12,014 HE. Copernicus published De revolutionibus in 11,543 HE. This way there is no need for a complicated conversion. Admittedly, this choice is arbitrary. We know of no specific historical event happened in 10,000 BCE. We will never know of one, because there was no writing at that time. It being arbitrary is the best part about it. Even Göbekli Tepe is in the Holocene Era. The Great Pyramid was finished in 7461 HE. Your choice, Copernicus, forces us to put too many historical events in the period before the calendar, keeping needless negative years. Do we really want to say that the Ottomans conquered Constantinople in 90 BC (before Copernicus)? * Or, to be more hip, update it to call it the Anthropocene Era. Note: not an atheist, and you don't have to be to like this calendar.