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
The Mammertines Received into the Italian Confederacy
It was a moment of the deepest significance in the history of the world, when the envoys of the Mamertines appeared in the Roman senate. No one indeed could then anticipate all that was to depend on the crossing of that narrow arm of the sea; but that the decision, however it should go, would involve consequences far other and more important than had attached to any decree hitherto passed by the senate, must have been manifest to every one of the deliberating fathers of the city.
Strictly upright men might indeed ask how it was possible to deliberate at all, and how any one could even think of suggesting that the Romans should not only break their alliance with Hiero, but should, just after the Campanians of Rhegium had been punished by them with righteous severity, admit the no less guilty Sicilian accomplices to the alliance and friendship of the state, and thereby rescue them from the punishment which they deserved. Such an outrage on propriety would not only afford their adversaries matter for declamation, but must seriously offend all men of moral feeling.
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Reference address : https://www.ellopos.net/elpenor/rome/3-02-war-rome-carthage-sicily.asp?pg=7