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
To all appearance the Celtic nation, when Caesar encountered it, had already reached the maximum of the culture allotted to it, and was even now on the decline. The civilization of the Transalpine Celts in Caesar's time presents, even for us who are but very imperfectly informed regarding it, several aspects that are estimable, and yet more that are interesting; in some respects it is more akin to the modern than to the Greek-Roman culture, with its sailing vessels, its knighthood, its ecclesiastical constitution, above all with its attempts, however imperfect, to build the state not on the city, but on the tribe and in a higher degree on the nation.
But just because we here meet the Celtic nation at the culminating point of its development, its lesser degree of moral endowment or, which is the same thing, its lesser capacity of culture, comes more distinctly into view. It was unable to produce from its own resources either a national art or a national state; it attained at the utmost a national theology and a peculiar type of nobility.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=41