Before the current and prevailing tri-partite model of history with Ancient, Medieval and Modern periods, there was a prevailing bi-partite view of history; before and after the birth of Christ.
And this is poetically described in Dante’s Purgatory 10:34-36 here:
The angel who came down to announce on earth
the peace longed for by weeping centuries,
which broke the ancient ban and opened Heaven
pointing to the Annunciation with the Angel Gabriel as the point where history changed. This is also the first image of Humility, on the First Terrace of Mount Purgatory and the starting point for the inner change, towards Virtue.