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
In this way, while the two great Roman armies, each in itself equal in numbers to that of Hannibal, remained at a great distance from the valley of the Po, the Romans were quite unprepared for an attack in that quarter. No doubt a Roman army was there, in consequence of an insurrection that had broken out among the Celts even before the arrival of the Carthaginian army.
The founding of the two Roman strongholds of Placentia and Cremona, each of which received 6000 colonists, and more especially the preparations for the founding of Mutina in the territory of the Boii, had already in the spring of 536 driven the Boii to revolt before the time concerted with Hannibal; and the Insubres had immediately joined them. The colonists already settled in the territory of Mutina, suddenly attacked, took refuge in the town.
The praetor Lucius Manlius, who held the chief command at Ariminum, hastened with his single legion to relieve the blockaded colonists; but he was surprised in the woods, and no course was left to him after sustaining great loss but to establish himself upon a hill and to submit to a siege there on the part of the Boii, till a second legion sent from Rome under the praetor Lucius Atilius succeeded in relieving army and town, and in suppressing for the moment the Gaulish insurrection.
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Reference address : https://www.ellopos.net/elpenor/rome/3-05-war-hannibal-cannae.asp?pg=2