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 while the new classicism--that is to say, the standard Latin governed by rule and as far as possible placed on a parity with the standard Greek-- which arose out of a conscious reaction against the vulgarism intruding into higher society and even into literature, acquired literary fixity and systematic shape, the latter by no means evacuated the field. Not only do we find it naively employed in the works of secondary personages who have drifted into the ranks of authors merely by accident, as in the account of Caesar's second Spanish war, but we shall meet it also with an impress more or less distinct in literature proper, in the mime, in the semi-romance, in the aesthetic writings of Varro; and it is a significant circumstance, that it maintains itself precisely in the most national departments of literature, and that truly conservative men, like Varro, take it into protection.
Classicism was based on the death of the Italian language as monarchy on the decline of the Italian nation; it was completely consistent that the men, in whom the republic was still living, should continue to give to the living language its rights, and for the sake of its comparative vitality and nationality should tolerate its aesthetic defects. Thus then the linguistic opinions and tendencies of this epoch are everywhere divergent; by the side of the old-fashioned poetry of Lucretius appears the thoroughly modern poetry of Catullus, by the side of Cicero's well-modulated period stands the sentence of Varro intentionally disdaining all subdivision. In this field likewise is mirrored the distraction of the age.
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Reference address : https://www.ellopos.net/elpenor/rome/5-12-religion-culture-literature-art.asp?pg=21