In this paper I will argue first that insufficient notice has been given to Dio 57.20.1–22.4 (preserved by Xiphilinus) and that, by having ignored Dio's general chronology and trusting to that provided by Tacitus' Annals, we have been led astray in determining the precise chronology of A.D. 21, and, therefore, the movements of Tiberius in that year. Having established the bona fides of Dio's (Xiphilinus') account, I will argue for a revised chronology of A.D. 21, in particular that, although the princeps Tiberius left Rome for Campania in January A.D. 21, he returned to the capital about three months later, in April A.D. 21, not in April A.D. 22, as most believe. From that point, I will argue that the return of Tiberius was connected to the illness of his son Drusus, at the very least in a temporal sense, and that this event occurred before or during May. Integral to this reinterpretation of Tiberius' movements this year is an understanding of the way the Tacitus has treated the consulship of 21. The short length of Tiberius' sojourn in Campania and his return to Rome in April clearly have important ramifications for the interpretation of the other events of A.D. 21. Although there are many issues that arise from a reinterpretation of Tiberius' movements in A.D. 21, I will focus only on the Gallic Revolt. Tacitus has this event apparently begin and end in the period of Tiberius' absence from Rome in A.D. 21, in a mere three-month period, which cannot have been the case. I will examine the evidence for the tenure of the revolt, then I will explore Tacitus' literary, rather than historical purpose in detailing the role of the emperor in these events.