The 15th Year of Tiberius Caesar
The year of Augustus' death is firmly fixed and confirmed by a Lunar eclipse recorded by the Roman historian Tacitus (Annals, Book 1, paragraph 28). However, Edward Gibbon (Decline and Fall of the Roman Empire, vol 1, p. 30) notes that prior to his death Augustus "dictated a law by which the future prince [in this case Tiberius] was invested with an authority equal to his own, over the provinces and the armies."
Within "the provinces" Tiberius had equal standing and authority with Caesar Augustus even while Augustus was still alive. Historian Will Durant points out that "all Rome which hated [Tiberius] for his stern puritanism, resigned itself to the fact that though Augustus was still prince, Tiberius had begun to rule." (Durant, The Story of Civilation, vol. 3 "Caesar and Christ" p. 231)
Thus the reign of Tiberius is dated (not from his sole reign at the death of Augustus) but from the beginning of his joint reign (two years prior to his death in A.D. 12) when Tiberius was given legal equality with Augustus. The 15th year if Tiberius would then correspond to A.D. 27.