The general chronology of the reign of Decius has been described by Arthur Stein in a recent paper, based principally on the evidence of papyri, inscriptions and coins.
Grenfell and Hunt had already pointed out (Ox. Pap. i, 35), in connexion with a papyrus giving a list of emperors from Augustus to Decius with the number of years of each reign, that ‘in reckoning the length of reigns, the months after the last Thoth I (Aug. 29) in an emperor's reign are neglected, since the interval between the death of an emperor and the next Thoth I counted as the 1st year of his successor.’ The latter half of the reign of Decius extended as we shall see for nearly a year after Aug. 29, A.D. 250. As, therefore, he has only two years (α and β) both on the papyri and on the coins of the Alexandrian mint, it follows that his reign in Egypt begins at a date after Aug. 29, 249. In other words, although Decius had been proclaimed emperor by the Pannonian legions at the close of 248 or in the first part of 249, the authority of Philip was maintained in Egypt until his overthrow and death at Verona in the autumn of 249. This is confirmed by the dating of Philip's coins which enter on a seventh year in Egypt and by Pap. Brit. Mus. 950-1 which, however, gives no month.