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
Cato the Elder more briefly describes them, nearly to the same effect; "the Celts devote themselves mainly to two things--fighting and -esprit-."(6)
6. -Pleraque Gallia duas res industriosissime persequitur: rem militarem et argute loqui- (Cato, Orig, l. ii. fr. 2. Jordan).
Such qualities--those of good soldiers but of bad citizens--explain the historical fact, that the Celts have shaken all states and have founded none. Everywhere we find them ready to rove or, in other words, to march; preferring moveable property to landed estate, and gold to everything else; following the profession of arms as a system of organized pillage or even as a trade for hire, and with such success at all events that even the Roman historian Sallust acknowledges that the Celts bore off the prize from the Romans in feats of arms.
They were the true soldiers-of-fortune of antiquity, as figures and descriptions represent them: with big but not sinewy bodies, with shaggy hair and long mustaches--quite a contrast to the Greeks and Romans, who shaved the head and upper lip; in variegated embroidered dresses, which in combat were not unfrequently thrown off; with a broad gold ring round the neck; wearing no helmets and without missile weapons of any sort, but furnished instead with an immense shield, a long ill-tempered sword, a dagger and a lance--all ornamented with gold, for they were not unskilful at working in metals.
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Reference address : http://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=11