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
Fruitlessness of the Celtic Victory
The fearful catastrophe of the defeat and the conflagration, the 18th of July and the rivulet of the Allia, the spot where the sacred objects were buried, and the spot where the surprise of the citadel had been repulsed--all the details of this unparalleled event--were transferred from the recollection of contemporaries to the imagination of posterity; and we can scarcely realize the fact that two thousand years have actually elapsed since those world-renowned geese showed greater vigilance than the sentinels at their posts.
And yet --although there was an enactment in Rome that in future, on occasion of a Celtic invasion no legal privilege should give exemption from military service; although dates were reckoned by the years from the conquest of the city; although the event resounded throughout the whole of the then civilized world and found its way even into the Greek annals--the battle of the Allia and its results can scarcely be numbered among those historical events that are fruitful of consequences.
It made no alteration at all in political relations. When the Gauls had marched off again with their gold--which only a legend of late and wretched invention represents the hero Camillus as having recovered for Rome--and when the fugitives had again made their way home, the foolish idea suggested by some faint-hearted prudential politicians, that the citizens should migrate to Veii, was set aside by a spirited speech of Camillus; houses arose out of the ruins hastily and irregularly--the narrow and crooked streets of Rome owed their origin to this epoch; and Rome again stood in her old commanding position.
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Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=25