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
But they had no knowledge of the art of besieging, and the blockade of the steep citadel rock was tedious and difficult, because subsistence for the great host could only be procured by armed foraging parties, and the citizens of the neighbouring Latin cities, the Ardeates in particular, frequently attacked the foragers with courage and success.
Nevertheless the Celts persevered, with an energy which in their circumstances was unparalleled, for seven months beneath the rock, and the garrison, which had escaped a surprise on a dark night only in consequence of the cackling of the sacred geese in the Capitoline temple and the accidental awaking of the brave Marcus Manlius, already found its provisions beginning to fail, when the Celts received information as to the Veneti having invaded the Senonian territory recently acquired on the Po, and were thus induced to accept the ransom money that was offered to procure their withdrawal.
The scornful throwing down of the Gallic sword, that it might be outweighed by Roman gold, indicated very truly how matters stood. The iron of the barbarians had conquered, but they sold their victory and by selling lost it.
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Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=24