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
To the same class belongs the history of the foundation of the city itself, which was designed to connect the origin of Rome with Latium and with Alba, the general metropolis of the Latins. Historical glosses were annexed to the surnames of distinguished Romans; that of Publius Valerius the "servant of the people" (-Poplicola-), for instance, gathered around it a whole group of such anecdotes. Above all, the sacred fig-tree and other spots and notable objects in the city were associated with a great multitude of sextons' tales of the same nature as those out of which, upwards of a thousand years afterwards, there grew up on the same ground the Mirabilia Urbis.
Some attempts to link together these different tales--the adjustment of the series of the seven kings, the setting down of the duration of the monarchy at 240 years in all, which was undoubtedly based on a calculation of the length of generations,(16) and even the commencement of an official record of these assumed facts--probably took place already in this epoch.
16. They appear to have reckoned three generations to a hundred years and to have rounded off the figures 233 1/3 to 240, just as the epoch between the king's flight and the burning of the city was rounded off to 120 years (Cf. II. IX. Registers of Magistrates, note). The reason why these precise numbers suggested themselves, is apparent from the similar adjustment (above explained, Cf. I. XIV. The Duodecimal System) of the measures of surface.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=15