Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
He rid himself--in a perfidious manner, it is true--of the insubordinate army of mercenaries, revived the citizen- militia, and endeavoured, at first with the title of general, afterwards with that of king, to re-establish the deeply sunken Greek power by means of his civic troops and of fresh and more manageable recruits. With the Carthaginians, who in concert with the Greeks had driven king Pyrrhus from the island, there was at that time peace. The immediate foes of the Syracusans were the Mamertines.
They were the kinsmen of those hated mercenaries whom the Syracusans had recently extirpated; they had murdered their own Greek hosts; they had curtailed the Syracusan territory; they had oppressed and plundered a number of smaller Greek towns. In league with the Romans who just about this time were sending their legions against the Campanians in Rhegium, the allies, kinsmen, and confederates in crime of the Mamertines,(2) Hiero turned his arms against Messana.
2. Cf. II. VII. Submission of Lower Italy
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Reference address : https://www.ellopos.net/elpenor/rome/3-02-war-rome-carthage-sicily.asp?pg=5