There shouldn't be any huge reason for Porre to change the calandar, as far as I'm concerned. Just because an empire falls, donesn't mean you gotta change the system. The roman empire fell how long ago? It was like 40 A.D., right? No one seemed too eager to change it.
*slaps forehead* Gotta love our education system...
The Roman Empire did
not fall in 40 AD. It was just getting to the good part around then.
When the Roman Empire fell depends on what you see as the "fall." Several smaller empires broke away circa 260 AD, but were recovered around 290. However, the Roman government split around then into the Eastern and Western empires. This could be called the fall of the empire.
Wait just a few years to 359 AD, and you could also call when the Emperor Constantine moved the entire Roman government from Rome to his new city of Constantinople in modern Turkey the "fall of the Roman Empire."
Or, if you want to just count when the last Emperor fell, that wouldn't be until 1453 AD, although the empire at that point had become known as the Byzantine Empire.
And, to cap all this off, the Romans didn't even use the BC/AD system. They counted from the founding of Rome. Year 1 ad urbe condita (Year 1 since the founding of the City) was about the year 753 BC according to their legends. The dating system was changed by a bunch of monks sometime during the Renaissance.