General:
Years are counted from the semi-mythical founding of the Crystal City (which was at first called Lake Hill, and was then on the great lake which once occupied the majority of central northern North Amareica). This happened 447 years after the Cataclysm. The founders, who as the descendants of a university faculty knew about the computational problems of starting with a year 1, declared their first year to be "zero," facilitating conversions, so year 447 ATC is year 0 AUC (Anno Urbis
Ponies who have been reading Corruption at Nightfall may have noticed that the rulers of the Crystal Empire are titled "Emperors" rather than "Kings." This is because the Crystal Empire was once a Republic, and the title of "emperor" evolved from one given to their chief magistrate, who was commander of their armies. It is exactly cognate, allowing for linguistic drift, to the title of "Chief Commander" used by the Pegasi of Equestria.