That's the problem, "the Fifteenth year of Tiberius" would not have meant a thing to anybody in Greece or Italy either. Historical writers of the time simply did not date things that way.
Suetonius, in writing on the Life of Tiberius tells many things that Tiberius did during his reign. For example:
There is nothing that tells which year of his reign Tiberius did this in. Only once does Suetonius tell a fact and connect it with the years of Tiberius' reign. He writes, " [38]For two whole years after becoming emperor he did not set foot outside the gates; after that he went nowhere except to the neighboring towns, at farthest to Antium, and even that very seldom and for a few days at a time." This is just to indicate how he disliked traveling. It is not dating anything by the years of his reign.He abolished foreign cults, especially the Egyptian and the Jewish rites, compelling all who were addicted to such superstitions to burn their religious vestments and all their paraphernalia. Those of the Jews who were of military age he assigned to provinces of less healthy climate, ostensibly to serve in the army; the others of that same race or of similar beliefs he banished from the city, on pain of slavery for life if they did not obey. He banished the astrologers as well, but pardoned such as begged for indulgence and promised to give up their art.
Cassius Dio, in writing about Tiberius in books 57 and 58 of his History, written in about 230 CE, only uses the names of consuls to date years. Only once does he mention an event and names the year of Tiberius' reign, but he quickly connects it to the names of the consuls for that time: "The twentieth year of Tiberius' reign was now at hand, but he did not enter the city, although he was sojourning in the vicinity of the Alban territory and Tusculum; the consuls, however, Lucius Bitellius and Fabius Persicus, celebrated the completion of his second ten-year period."
Since it appears that it was not the practice of historians of the First, Second or Early Third century to date events according to the years of Tiberius' reign, we are left with the puzzle of why Luke does it.
Andrew wrote:If Luke was writing to the Gentiles (was he?) then that might make sense, since the names of Pilate or Archelaus may not have meant anything to someone in Greece or Italy, for instance, but the name of Tiberius would.PhilosopherJay
We have to consider Luke's reference to the reign of Tiberius as odd. Who would have known or cared what happened in the fifteenth year of Tiberius' reign? it would make sense only in a book about the reign of Tiberius or the reign of emperors of that time. Since Luke was talking about events in Judea, it would have made sense for him to give the year of Pilate's reign or Archelaus' reign. It makes no sense for him to give the year of Tiberius' reign if he was trying to give a real historically useful time marker. It only makes sense as a faux (made-up, pretend) time marker.
Warmly,
Jay Raskin