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
The New Roman Poetry
They soon went farther. What Cicero did in prose, was carried out in poetry towards the end of the epoch by the new Roman school of poets, which modelled itself on the Greek fashionable poetry, and in which the man of most considerable talent was Catullus. Here too the higher language of conversation dislodged the archaic reminiscences which hitherto to a large extent prevailed in this domain, and as Latin prose submitted to the Attic rhythm, so Latin poetry submitted gradually to the strict or rather painful metrical laws of the Alexandrines; e. g. from the time of Catullus, it is no longer allowable at once to begin a verse and to close a sentence begun in the verse preceding with a monosyllabic word or a dissyllabic one not specially weighty.
At length science stepped in, fixed the law of language, and developed its rule, which was no longer determined on the basis of experience, but made the claim to determine experience. The endings of declension, which hitherto had in part been variable, were now to be once for all fixed; e. g. of the genitive and dative forms hitherto current side by side in the so-called fourth declension (-senatuis- and -senatus-, -senatui-, and -senatu-) Caesar recognized exclusively as valid the contracted forms (-us and -u). In orthography various changes were made, to bring the written more fully into correspondence with the spoken language; thus the -u in the middle of words like -maxumus- was replaced after Caesar's precedent by -i; and of the two letters which had become superfluous, -k and -q, the removal of the first was effected, and that of the second was at least proposed.
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Reference address : https://www.ellopos.net/elpenor/rome/5-12-religion-culture-literature-art.asp?pg=19