Of time and ill chance

For my second question-answering post, a pair of in-world questions – thank you NeoAC!

The first is about dates, or more accurately, about how years are accounted for. We know that Lachazzar calls for the Crusade in the year 2781 of the calenda annalis, and other years pop there and here, starting with the ones that are mentioned in Carausius's History. I always wondered what event marked the year one of that era. Something like Romans, when they used the Ab urbe condita system, that's it...