On a Gospel Without the Reference 'In the Fifteenth Year of Tiberius'

It is unusual that only one gospel makes reference to the actual dating of the gospel narrative.  Of course there is the additional claim that the Marcionite gospel also had these words.  But we can't be sure of that. To be certain there was a conscious effort to make it seem that these words were in their gospel too.  Nevertheless this is not decisive in itself.  Moreover, it is unusual that neither Mark nor Matthew nor John make reference to this statement.  How can that be explained?

1. The words were in the other synoptic gospels but later removed.
2. The words were in Marcion but removed by the other synoptic gospels
3. Luke 'discovered' the beginning of Jesus's ministry and Marcion copied him.

I don't find any of these arguments particularly persuasive.  How can the addition of the statement 'in the fifteenth year of Tiberius' be explained?  

Clement recognizes that the words appeared only in Luke.  Most people don't interpret what Clement says about Luke's translation efforts of the Epistle to the Hebrews (Eusebius, History of the Church 6.14.1-4) as a negative statement.  Clement uses the word 'honorably' to describe his efforts.  Nevertheless Luke must have been thought to have only 'loosely' translated the original material here because Clement makes reference to the fact that much of the word reads like Acts which the Alexandrian presumes Luke wrote.

The most obvious explanation to why the Gospel of Luke has 'in the fifteenth year of Tiberius' is because Luke found it somewhere other than Mark, Matthew or John and reproduced it here. Epiphanius says:

For the canon of Luke is revelatory of their form of the Gospel: mutilated as it is, without beginning, middle or end, it looks like a cloak full of moth holes.  At the very beginning he excised everything Luke had originally composed—his 'inasmuch as many have taken in hand,' and so forth, and the material about Elizabeth and the angel's announcement to Mary the Virgin; about John and Zacharias and the birth at Bethlehem; the genealogy and the story of the baptism. All this he cut out and turned his back on, and made this the beginning of the Gospel, 'In the fifteenth year of Tiberius Caesar,' and so on.  

But as we have noted many times, Epiphanius was working off someone else's account of the shape of the Marcionite gospel - and that original author was certainly Irenaeus.  I am not sure that the Marcionite text ever had 'in the fifteenth year of Tiberius.'

Indeed if we look through the actual Latin text of Tertullian's Against Marcion there is no reason to believe the words were there.  We read for instance:

As corrector apparently of a gospel which from the times of Tiberius to those of Antoninus had suffered subversion, Marcion comes to light, first and alone, after Christ had waited for him all that time, repenting of having been in a hurry to send forth apostles without Marcion to protect them. [AM 4.5]

Marcion lays it down that there is one Christ who in the time of Tiberius was revealed by a god formerly unknown, for the salvation of all the nations; and another Christ who is destined by God the Creator to come at some time still future for the re-establishment of the Jewish kingdom.[4.7]

In the fifteenth year of the principate of Tiberius he puts forward he came down into Capernaum, a city of Galilee—from the Creator's heaven, of course, into which he had first come down out of his own [4.8]

Irenaeus and Hippolytus also:

However (it) was that, independent of birth, (the Logos) Himself descended from above in the fifteenth year of the reign of Tiberius Caesar, and that, as being intermediate between the good and bad Deity, He proceeded to give instruction in the synagogues. [Philosophumena 7.19]

Marcion of Pontus succeeded him, and developed his doctrine. In so doing, he advanced the most daring blasphemy against Him who is proclaimed as God by the law and the prophets, declaring Him to be the author of evils, to take delight in war, to be infirm of purpose, and even to be contrary to Himself. But Jesus being derived from that father who is above the God that made the world, and coming into Judaea in the times of Pontius Pilate the governor, who was the procurator of Tiberius Caesar, was manifested in the form of a man to those who were in Judaea, abolishing the prophets and the law, and all the works of that God who made the world, whom also he calls Cosmocrator. [Irenaeus 1.27.2]

Now if any man set Luke aside, as one who did not know the truth, he will, [by so acting,] manifestly reject that Gospel of which he claims to be a disciple. For through him we have become acquainted with very many and important parts of the Gospel; for instance, the generation of John, the history of Zacharias, the coming of the angel to Mary, the exclamation of Elisabeth, the descent of the angels to the shepherds, the words spoken by them, the testimony of Anna and of Simeon with regard to Christ, and that twelve years of age He was left behind at Jerusalem; also the baptism of John, the number of the Lord's years when He was baptized, and that this occurred in the fifteenth year of Tiberius Caesar. [Irenaeus 3.14.3]

Indeed I think it is Irenaeus who is the originator of this combative style of attacking the heretics.  In other words, Irenaeus has a consistent way of rhetorically backing his opponents into a corner.  He throws in false statements into the mix of his summary of his opponents position - in this case 'the fifteenth of Tiberius.'

Why would Irenaeus add the bit about the fifteenth of Tiberius?  Well just think about it for a moment.  His purpose is to make it seem ridiculous that anyone would have expected God to come down from heaven in a particular year.  In other words, there is a deliberate effort to strip away anything that would make the idea of a theophany seem plausible or reasonable.  We know from the Qumran documents that Jews expected this would happen in a Jubilee year.  The opening words of the gospel as the Basilideans knew it happened to start the gospel on the day that ancient Israelites calculated the proper starting date for the new year.  It also betrayed the sectarian or even Egyptian provenance of the original gospel writer.  If the reader pays careful attention, there is absolutely nothing in the existing gospel which gives any clue as to the partisan affiliation of the original gospel writer.

While Luke makes reference to the 'Good Samaritan' most of the Samaritan related material is actually buried in the Gospel of John, I am not certain this was accidental or natural.