Implicit in the mention of the precise date of a mythological event is the claim that someone witnessed the descent of Jesus Son of Father ("Bar-Abbas") on the earth. For Marcion, that particular witness could only be: Paul.
The Judaizers interpolated John the Baptist in that same incipit:
(Luke 3:1-2)
Now, with the interpolation, John the Baptist assumes the role of the ocular witness of the descent of a mythical being ("the logos of god").
This means that in the original incipit of Marcion's Evangelion, the voice of god addressed Paul himself, with the words:
Since so we read in Galatians 1:15-16:
This explains why the incipit of Mark 1 is read easily by Tarazi, Dykstra inter alia as midrash on Paul.
My point is that Paul, in the original (but lost) marcionite incipit, figured as the original witness of the descent of Jesus on earth.
This explains why: 15° year of Tiberius.