No other surviving gospel makes this explicit reference to the year of Jesus's death. Mark says nothing about it, nor Matthew or John. If, as most scholars now assume, Luke took a narrative originally written by Mark and added a number of 'things' to it, 'the fifteenth of Tiberius' was certainly be one such detail. Yet is this how this critical detail made its way into our collective consciousness? Did Luke simply take a text without the reference to 'the fifteenth of Tiberius' and add this date to our canonical gospel of Mark?
If this is the way the addition manifested itself one can suppose that 'Luke' got his information from somewhere reliable. After all, 'Luke' makes reference to the kind of information that historians like to see represented in source material, things like censuses and historical chronologies. But the story isn't that simple. When Paul declares his association with a text he refers to as 'my gospel' could he really be imagining that an associate took a gospel associated with another Christian and then went to the library on his own initiative and 'added' historical details lacking in the original. I don't think so.
The closest we can get to understanding what Paul meant by 'my gospel' is that it was the result of some sort of heavenly revelation. We get this understanding from the earliest references associated with a group of Christian writers now called the 'early Church Fathers.' Of course the Church Fathers themselves as a rule accept the fact that Luke wrote on behalf of Paul and when Paul says 'my gospel' he means 'the gospel of Luke.' It should be noted that not all the early Church Fathers believed this, but it is as a rule a fair representation of what 'the official Church' based in Rome believed in the late second century.
Nevertheless we learn from sources around this time and later that there was an older understanding of how the gospel associated with 'Paul' emerged in history. This understanding is rather sketchy but basically follows the declaration of the apostle in the letter called 'to the Corinthians' (the first of two documents with this name in the orthodox canon). Here we learn that 'Paul' took a text or an understanding originally associated with Peter and the apostles and 'perfected' or polished it according to a revelation he received when he rose to the 'third heaven' in an out of body experience. This was probably the original understanding of what Paul meant when he made reference to 'my gospel' in his canonical letters.
In other words, it wasn't 'Luke' who added details to an oral tradition associated with Paul but rather Paul who added 'mystical' knowledge to an oral teaching associated with Peter. This gospel is now lost but the basic formula bears striking resemblance to what we learn about Mark's gospel writing activities in early sources. It is worth noting that one early source identifies a radical sect which strictly adhered to the doctrines and texts of 'Paul' as possessing a mystical gospel according to Mark. This group called 'the Marcionites' in the writings of the early Church Fathers will become critical in determining whether or not Luke was manipulating early source material in order to establish the wrong date for Jesus's crucifixion.
The Marcionites were an extremely ancient Christian sect. Many if not most scholars now believe that they established the first 'New Testament canon' - that is, a closed collection of writings assumed to be wholly made up of texts written by 'Paul.' But who was 'Paul'? Even the canonical 'Acts of the Apostles' says that 'Paul' wasn't his original name. Is it possible that rather than the homophone appellation 'Saul' the name or title 'Paul' developed as a way of disguising the apostle's real identity as Mark the companion of Peter and more importantly the understanding that his gospel - that is 'the gospel according to Mark' - was not sanctioned by his master Peter? We shall revisit this idea later in our investigation.
Our present interest in merely establishing the most basic 'truth' about the gospel narrative - that is, when did it 'happen'? The gospel according to Luke, that text which is supposed to represent 'my gospel' - that is 'Paul's gospel' - and at the same time an expansion of the primitive gospel faithfully written down by Mark for Peter, is our only source for this critical historical information. So the next logical question emerges, whether or not 'the fifteenth of Tiberius' was already present in this text - that is, the orthodox version of the 'secret gospel' treasured by the Marcionite community and preserved by them as the original gospel 'according to Mark' (undoubtedly because 'Mark' was really 'Paul')?
After reviewing all the surviving evidence it will be determined that 'the fifteenth of Tiberius' was undoubtedly a deliberate misrepresentation of the original reading 'the fifteenth of Tybi' - that is, the fifth month in the Egyptian calendar. We know this from a reference made in the writings of a prominent Alexandrian Church Father from the turn of the third century named Clement. The reference was first noted by George Mead at the turn of the last century who cites the reference as follows:
"They of Basilides," says Clement, "celebrate His Baptism by a preliminary night-service of readings; and they say that 'the fifteenth year of Tiberius Caesar' means the fifteenth day of the month Tybi." It was then that the Father "in the likeness of a dove"--which they explained as meaning the Minister or Holy Spirit--came upon Him. In "the fifteenth [year] of Tib[erius]" we have, then, perhaps an interesting glimpse into the workshop of the "historicizers." [Fragments of a Faith Forgotten p. 278]
The original reference in the manuscripts of Clement make clear that the author is moving on from determining the date of Jesus's birth to that of his baptism and then we hear:
φασὶ δὲ εἶναι τὸ πεντεκαιδέκατον ἔτος Τιβερίου Καίσαρος τὴν πεντεκαιδεκάτην τοῦ Τυβὶ μηνός, τινὲς δὲ αὖ τὴν ἑνδεκάτην τοῦ αὐτοῦ μηνός
they say that the 'fifteenth year of Tiberius Caesar' the fifteenth day of the month Tybi while some the eleventh of the same month
To this end it would seem that Mead's interpretation of this passage is correct - at least one heretical group's gospel began with a reference to a specific day rather than a year - i.e. 'the fifteenth of Tybi' rather than 'the fifteenth of Tiberius' - leaving open the question of what the actual year wide open for speculation.