Years were often reckoned from the foundation of Rome ('ab urbe condita').
The traditional date for the foundation was the year that we now call 753 BC. In the later Roman empire years were sometimes numbered by reference to 'indictions' (15-year periods instituted by Diocletian for tax purposes).
Although it was the very greatest of the festivals of the Church, if he had known in advance what the Pope was planning to do.’ …
“Yet still an aura of mystery lingered around the ceremony.
Over the preceding decades, the great program of correctio had begun to embrace even the dimensions of time itself.
Almost all of the world's religious calendars are based on religion, astrology, or myth: The division between BC/BCE and AD/CE is not based on religious considerations.
Nothing of a religious nature happened during 1 BCE and 1 CE -- in fact nothing of truly momentous importance happened at all, to our knowledge.
He decided to start his 532-year cycle from 753 years after the Founding of Rome.
Dionysius did not want to perpetuate the name of Alexander, the Great Persecutor.