What calendar date was the basis pre-Aegon’s Conquest/Crowning? I think it may vary per kingdom. If you had to design one for each kingdom what would you have them be? 

North – The Wall being built? Northern Unification?

Vale – Andal Arrival or more likely Arryn Conquest

Iron Isles – Writing/time keeping is for Greenlanders

Riverlands – Varying between ruling dynasty

Westerlands – Lann the clever takes Casterly Rock?

Stormland – Durran Godsgrief’s reign?

Reach – Garth Greenhand’s Reign

Citidel – Founding of Citidel/Andal Invasion?

Dorne – Nymeria’s Conquest

Interesting question. I don’t think it varied by kingdom, because I’m pretty sure the maesters of the Citadel would have been in charge of the chronicles and thus the dating systems and would have wanted a uniform system.