"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]
Mead's point is that the Basilideans aren't simply saying that the baptism occurred on the 15th of Tybi but that this reading replaces the familiar 'fifteenth year of Tiberius Caesar' in Luke (or more like a Diatessaron-like gospel).
Mead isn't alone in his interpretation of the material this way. Henry Wace and William Coleman Piercy in their Dictionary of Christian Biography and Literature to the End of the Sixth Century (hardly a theosophical handbook) break it down as follows:
They of Basilides (οἱ ἀπὸ Β.) celebrate the day of His Baptism by a preliminary night-service of [Scripture] readings (προδιανυκτερεύοντες ἀναγνώσεσι); and they say that the 'fifteenth year of Tiberius Caesar' (Luk_3:1) is (or means) the fifteenth day of the [Egyptian] month Tybi while some [make the day] the eleventh of the same month.
or if the reader wants to see the whole passage:
Οἱ δὲ ἀπὸ Βασιλείδου καὶ τοῦ βαπτίσματος αὐτοῦ τὴν ἡμέραν ἑορτάζουσι προδιανυκτερεύοντες <ἐν> ἀναγνώσεσι. φασὶ δὲ εἶναι τὸ πεντεκαιδέκατον ἔτος Τιβερίου Καίσαρος τὴν πεντεκαιδεκάτην τοῦ Τυβὶ μηνός, τινὲς δὲ αὖ τὴν ἑνδεκάτην τοῦ αὐτοῦ μηνός. τό τε πάθος αὐτοῦ ἀκριβολογούμενοι φέρουσιν οἳ μέν τινες τῷ ἑκκαιδεκάτω ἔτει Τιβερίου Καίσαρος Φαμενὼθ κεʹ, οἳ δὲ Φαρμουθὶ κε [1.21.146.1 - 3]
And here is the contentious sentence:
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
So yes, Mead is on fairly reliable ground with his interpretation (which was probably taken from Wace and Piercy) Clement is reporting that the Basilideans argued for 'the fifteenth of Tubi' as the reading in the gospel instead of 'the fifteenth of Tiberius Caesar.'
The question that was raised by Monobazus is - why would this prove an Egyptian provenance for the gospel? The month of Tybi is fifth month of the Egyptian calendar. The reference might prove once and for all that the gospel was reporting on events in Judea from the perspective of an Alexandrian or the Alexandrian community. There is another possibility however which I suspect is more accurate than that. The reference might be the missing link to prove that the first Christians were members of the Samaritan Dosithean sect. The Dositheans were especially numerous in Alexandria so the two ideas might not be mutually exclusive.
The Dositheans were an arch-conservative sect. They maintained a calendar of twelve thirty day months. As Stanley Isser writes in his book on the subject "nowhere, not even in Jubilees, is there a calendar which assigns thirty days to each month with no adjustments. Only the ancient Egyptians had twelve months of thirty days plus five supplementary days in each year." Qirqisani does identify the Sadducees as holding the same system but this in no way poses a challenge to the idea that the Dositheans were preserving the inescapable idea that anyone who took seriously the idea that Moses was the author of the Pentateuch (a silly idea but one generally held in antiquity) it would be impossible not to assume that his calendar would be Egyptian.
There are no names of months in the Pentateuch. One can imaging that the Dositheans and their orthodox opponents read the same same text and applied it to two separate calendars. The same thing was going on in the various Jewish and later Christian sectarian communities. Yet it is interesting that one of our oldest heresies - the Montanists - also used a twelve month thirty day calendar.
The point however is that the more we look at the problem of the Basilidean claim that the gospel said 'on the fifteenth of Tybi' rather than 'the fifteenth of Tiberius Caesar' the more that we see the fingerprints of Samaritanism on the gospel. Both Jews and Samaritans venerate the fifteenth day of the eleventh month (relative to Passover). Yet in the Samaritan tradition the so-called tsimmoth specifically distanced from the Passover by exactly sixty days.
It is interesting to note also that the gospel times the fifteenth of Tybi to the 'conjunction' of two figures - Jesus and John the Baptist - just as the Samaritan tradition connects it with Moses and Aaron. Both pairs of men meet in the wilderness. Both Jewish and Samaritan sources connect the collection of the half shekel tax on the fifteenth of the eleventh month. But only the Samaritans employ the tsimmoth for the final calculation of the new calendar. In other words, when Jesus goes on in the opening lines of the gospel to 'announce the year of favor' he is doing so after the calculations from the conjoining of the sun and moon on that day to help determine the exact shape of the year .
