It's funny to see all the English-speaker arguing about all the BC-AD/BCE-CE thing. In France, we do not have that, only av. J.-C. and ap. J.-C. (avant Jésus-Christ and après Jésus-Christ, which mean before Jesus Christ and after Jesus Christ). The Anno Domini system is officially adopted in France but, in the reality, we only use the av./ap. J.-C. system (if you heard someone saying “2017 de l’ère commune” (2017 of the Common Era) he will look at you with big eyes… and on the other hand, sometimes we said “avant notre ère” (before our Era) for BC).
Anyway, I’m not really fond of having a calendar based on the Civ you play or the dominant religion of your civ. Because I like how Civ6 manage to let you create your own, measure-made empire and culture. If I want to found Catholicism with zen meditation, mosques and warrior monks, it’s up to me. If I want to build the Potato Palace in Uruk, it’s my choice. The best for me (and it will perfectly suit the will of the dev to make Civ6 more RP than ever, that and the timeline) would be some kind of “measure-made” calendar, created automatically with some requirements.
First of all, you won’t have a calendar if you have not research the Calendar tech (the same way in Rhye’s and Fall, where you were in Bronze Age, Stone Age and then…).
Then, once you have the tech, the year 1 would be calculated from a specific event of your empire (ideas: foundation of a religion, building of your first wonder, discovery of your first natural wonder, construction of the Government Plaza, discovering of your first relic or your first great work of anything, beginning of your first Golden Age or Heroic Age, or even foundation of your first city if you have nothing else). In a RP way, it would be logical to choose an event that happened before the research of Calendar, but it will reduce the possibilities.
If a culture become dominant in a civ, it adopts the calendar of the dominant civ.
I think it could add flavour to the game while personalizing each play.