Using the four gospels to determine Jesus’ birth, however, is problematic since the Gospel of John does not agree with the other three and Matthew, Mark, and Luke do not always agree with each other regarding significant events. Cargill explains: According to the Gospel of Matthew, Jesus was born during the reign of Herod the Great.
Thus, the BC/AD system is fundamentally flawed in that it misrepresents the birth of Jesus by approximately 7 years.
This means that Jesus’ ministry did not begin around the year 30, but instead around the year 23.
Dionysius himself did not know when Jesus was born and his system makes no claims at dating that event definitively.
He seems to have arrived at his calculations through a reliance on scripture and known history of the time to create a calendar which would be acceptable to both the western and eastern churches of the time in harmonizing the celebration of Easter.