Actually when that information first surfaced (in a DVD 1 extra where Lelouch gives a history lesson to Suzaku), it said nothing about
Augustus, but explicitly mentioned
Julius Caesar and stated that the ATB calendar started the year Caesar attempted to invade Great Britain - an endeavor that failed when Eowyn rallied the Briton tribes around him for a counterattack. Granted, it's also possible 1 ATB began the year Eowyn
defeated the Romans, which could be a little later than 55 BC, but both of Caesar's invasions were quite short-lived (a year each IIRC).
In real life, Caesar initiated both of his (equally unsuccessful) campaigns in 55 and 54 BC. Augustus would follow in Caesar's footsteps, but only twenty years later, in 34, 27 and 25 BC.
Watch it here.
EDIT: oh ffs, now they changed Eowyn to Alwin? Come on... Screw these retcons.