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
Accordingly, if the calendar years were to be reckoned by this list of consuls, it was necessary to note the days of entering on and of demitting office in the case of each pair, along with such -interregna- as occurred; and this too may have been early done.
But besides this, the list of the annual magistrates was adjusted to the list of calendar years in such a way that a pair of magistrates were by accommodation assigned to each calendar year, and, where the list did not suffice, intercalary years were inserted, which are denoted in the later (Varronian) table by the figures 379, 383, 421, 430, 445, 453. From 291 u. c. (463 B. C.) the Roman list demonstrably coincides, not indeed in detail but yet on the whole, with the Roman calendar, and is thus chronologically certain, so far as the defectiveness of the calendar itself allows.
The 47 years preceding that date cannot be checked, but must likewise be at least in the main correct.(12) Whatever lies beyond 245 remains, chronologically, in oblivion.
12. The first places in the list alone excite suspicion, and may have been subsequently added, with a view to round off the number of years between the flight of the king and the burning of the city to 120.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=9