- 'the fifteenth of Tybi' is a recognized variant of 'the fifteenth of Tiberius' and
- Marcion is explicitly associated with the Egyptian calendar of twelve months (which includes 'Tybi') by Tertullian (Adv Marc 1.19)
It supports the idea that the Marcionite reading here was 'the fifteenth of Tybi.' Indeed look at the context of the passage from Tertullian:
For the time it must suffice to follow up bur present argument so far as to prove, and that in few words, that Christ Jesus is the representative of no other god than the Creator. 'In the fifteenth year of Tiberius Caesar Christ Jesus vouchsafed to glide down from heaven, a salutary spirit.' In what year of the elder Antoninus the pestilential breeze (aura canicularis) of Marcion's salvation, whose opinion this was, breathed out from his own Pontus, I have forborne to inquire. But of this I am sure, that he is an Antoninian heretic, impious under Pius. Now from Tiberius to Antoninus there are a matter of a hundred and fifteen and a half years and half a month.
Tertullian is following Irenaeus is trying to establish Marcion as a heretic who can be dated to the middle of the second century. However our other sources make clear he comes from a much earlier period - undoubtedly the first century.
Tertullian's point is that Jesus was identified in Marcion's gospel as coming according to the rising of the Dog-star Sirius rather than 'the fifteenth of Tiberius' as in Luke. The fifteenth of Tybi is also identified as the date which the Ascension occurred according to the Pistis Sophia. Given that the Egyptian tradition explicitly identifies a single year to Jesus's ministry (not only Clement and Origen but Irenaeus's attack against 'heresies' which certainly included Marcion) the fifteenth of Tybi was both the start and the end of the gospel.
The early Egyptian gnostic text the Pistis Sophia also tells us that the Egyptian lunar calendar was meant here because the fifteenth is also identified with a full moon. Given that the Hebrew calendar began in the spring the equivalent of 'the fifteenth of Tybi' would be the fifteenth of Kislev. Why is that significant? Because a gospel which announced the coming of Jesus from heaven on this date would clearly connect the Christian message with the 'abomination of desolation' both in Daniel 9:24 - 27 and more specifically with the events of Hanukkah.
For in 1 Maccabees we explicitly read the date of the fulfillment of Daniel's Seventy Weeks prophesy as taking place in the very same date:
Now the fifteenth day of the month Kislev, in the hundred forty and fifth year, they set up the abomination of desolation upon the altar, and builded idol altars throughout the cities of Judah on every side. (1 Maccabees 1:54)
We shall discuss the implications of this new paradigm over the next few days, but the obvious point of departure here is to revisit the underlying point of Daniel's expectation. Everyone agrees that Antiochus Epiphanes's actions profaned the temple. The Maccabean apologists claim that they managed to 'cleanse' the altar of impurities. But clearly not every Jewish group agreed.
Even though Daniel 9:24 - 27 was clearly written for and after the events associated with Antiochus Epiphanes the normative interpretation of Jews and Christians now is that it applies to the capture of Jerusalem in 70 CE. Why is that? Because Daniel's prophesy clearly doesn't 'jibe' with the positive message of the Maccabean propagandists. In other words, there must have been a group that did not think the temple had been purified or even that it was still in a state of impurity. The Maccabeans acted as is everything had been 'fixed' in spite of Antiochus's actions (much like optimists continuing to invest in the stock market after the events of 2008). Nevertheless a hard core group of critics (probably marginalized priests after the Maccabean and Hasmonean eras) argued that the altar was no longer sacred and without a holy place sacrifices could no longer take place.
The gospel story then and the specific Marcionite interpretation of that original text necessarily begin with Daniel's understanding - written after the events of 168 CE - that sacrifices could no longer take place in Jerusalem.