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 the construction of his epic was defective; indeed it must have been very lax and indifferent, when it was possible for the poet to insert a special book by way of supplement to please an otherwise forgotten hero and patron. On the whole the Annals were beyond question the work in which Ennius fell farthest short of his aim. The plan of making an Iliad pronounces its own condemnation. It was Ennius, who in this poem for the first time introduced into literature that changeling compound of epos and of history, which from that time up to the present day haunts it like a ghost, unable either to live or to die. But the poem certainly had its success.
Ennius claimed to be the Roman Homer with still greater ingenuousness than Klopstock claimed to be the German, and was received as such by his contemporaries and still more so by posterity. The veneration for the father of Roman poetry was transmitted from generation to generation; even the polished Quintilian says, "Let us revere Ennius as we revere an ancient sacred grove, whose mighty oaks of a thousand years are more venerable than beautiful;" and, if any one is disposed to wonder at this, he may recall analogous phenomena in the successes of the Aeneid, the Henriad, and the Messiad.
A mighty poetical development of the nation would indeed have set aside that almost comic official parallel between the Homeric Iliad and the Ennian Annals as easily as we have set aside the comparison of Karschin with Sappho and of Willamov with Pindar; but no such development took place in Rome. Owing to the interest of the subject especially for aristocratic circles, and the great plastic talent of the poet, the Annals remained the oldest Roman original poem which appeared to the culture of later generations readable or worth reading; and thus, singularly enough, posterity came to honour this thoroughly anti- national epos of a half-Greek -litterateur- as the true model poem of Rome.
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Reference address : https://www.ellopos.net/elpenor/rome/3-14-literature-art.asp?pg=83