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
No era was formed for ordinary use; but in ritual matters they reckoned from the year of the consecration of the temple of the Capitoline Jupiter, from which the list of magistrates also started.
The idea naturally suggested itself that, along with the names of the magistrates, the most important events occurring under their magistracy might be noted; and from such notices appended to the catalogue of magistrates the Roman annals arose, just as the chronicles of the middle ages arose out of the memoranda marginally appended to the table of Easter. But it was not until a late period that the pontifices formed the scheme of a formal chronicle (-liber annalis-), which should steadily year by year record the names of all the magistrates and the remarkable events.
Before the eclipse of the sun noticed under the 5th of June 351, by which is probably meant that of the 20th June 354, no solar eclipse was found recorded from observation in the later chronicle of the city: its statements as to the numbers of the census only begin to sound credible after the beginning of the fifth century,(13) the cases of fines brought before the people, and the prodigies expiated on behalf of the community, appear to have been regularly introduced into the annals only after the second half of the fifth century began.
13. Cf. I. VI. Time and the Occasion of the Reform, II. VII. System of Government
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=10