Until what time was the Ancient Roman calendar era, based on the founding of Rome, used? Or what was the latest historical event that was mentioned in chronicles with an Ab Urbe Condita year?
I was keen on finding a similar answer (or, at least, question) to the Eastern Roman world which led me to read a bit on the AUC timekeeping, and from what Wiki says, the AUC was never rightly an official way of measuring time, but rather a method to refer to special years and such. This changed when the Principes found it a convenient way to highlight the 800th birthday, and such events, and I guess it can have become more common in usage.
The official Republican calendar was based on the two serving consuls, i.e., in the Year of Consul 1 and Consul 2, this happened. Hence, this should rightly be considered the 'ancient Roman calendar era' with the AUC system a secondary measure which arose later in the Imperial time.
A trivial answer for the second bit would highlight that Livius' chronicle goes up to 9 BC, and hence the last year he definitively refers to as an AUC is 762. I am unaware of other chroniclers preferring this system to other calendars, but I am sure someone else can posit a better guess to a final year that was mentioned in any chronicle as an AUC.