Most theologians and religious historians believe that Yeshua of Nazareth (Jesus) was born during a Fall -- or less likely during a Spring, sometime between 7 and 4 BCE.
He used it to identify the years in the Easter tables that he prepared.
He did not use the notation to date historical events.
"Dionysius implied, but never stated, that Jesus was born 25 December 1 BC." The basis on which he linked the divide between BC and AD to the birth of Jesus is unknown.
There is general agreement that he guessed incorrectly.