At the root of many doubts that obscure our understanding of the manner and timing of Tiberius' accession to the principate there lies a chronological puzzle whose solution is perhaps not altogether beyond conjecture. On what date fell the dies imperii of the new emperor? Three (perhaps four) precise, but incorrect, answers are provided by Josephus, Dio and Tertullian. The difficulty is not removed by the brief or tendentious narratives of Velleius, Tacitus and Suetonius which offer us no such date, or of Dio himself. It is the purpose of this paper to argue that Tiberius accepted the principate at the hands of the Senate on a date between 1st and 3rd September, A.D. 14, and that his reluctance to do so is unlikely to have been a consequence of the receipt of news of the Pannonian revolt.