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 Earlier History
The current story led with some measure of connection, though the connecting thread was but weak and loose through the regal period down to the institution of the republic; but at that point legend dried up; and it was not merely difficult but altogether impossible to form a narrative, in any degree connected and readable, out of the lists of magistrates and the scanty notices appended to them. The poets felt this most. Naevius appears for that reason to have passed at once from the regal period to the war regarding Sicily: Ennius, who in the third of his eighteen books was still describing the regal period and in the sixth had already reached the war with Pyrrhus, must have treated the first two centuries of the republic merely in the most general outline.
How the annalists who wrote in Greek managed the matter, we do not know. Cato adopted a peculiar course. He felt no pleasure, as he himself says, "in relating what was set forth on the tablet in the house of the Pontifex Maximus, how often wheat had been dear, and when the sun or moon had been eclipsed;" and so he devoted the second and third books of his historical work to accounts of the origin of the other Italian communities and of their admission to the Roman confederacy. He thus got rid of the fetters of chronicle, which reports events year by year under the heading of the magistrates for the time being; the statement in particular, that Cato's historical work narrated events "sectionally," must refer to this feature of his method.
This attention bestowed on the other Italian communities, which surprises us in a Roman work, had a bearing on the political position of the author, who leaned throughout on the support of the municipal Italy in his opposition to the doings of the capital; while it furnished a sort of substitute for the missing history of Rome from the expulsion of king Tarquinius down to the Pyrrhic war, by presenting in its own way the main result of that history--the union of Italy under the hegemony of Rome.
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Reference address : https://www.ellopos.net/elpenor/rome/3-14-literature-art.asp?pg=93