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 all appearance the institution of an organized book of annals, and--what was certainly associated with it--the revision (which we have just explained) of the earlier list of magistrates so as to make it a year-calendar by the insertion, where chronologically necessary, of intercalary years, took place in the first half of the fifth century. But even after it became a practically recognized duty of the -pontifex maximus- to record year after year campaigns and colonizations, pestilences and famines, eclipses and portents, the deaths of priests and other men of note, the new decrees of the people, and the results of the census, and to deposit these records in his official residence for permanent preservation and for any one's inspection, these records were still far removed from the character of real historical writings.
How scanty the contemporary record still was at the close of this period and how ample room is left for the caprice of subsequent annalists, is shown with incisive clearness by a comparison of the accounts as to the campaign of 456 in the annals and in the epitaph of the consul Scipio.(14)
14. Cf. II. VIII Rome and the Romans of this Epoch. According to the annals Scipio commands in Etruria and his colleague in Samnium, and Lucania is during this year in league with Rome; according to the epitaph Scipio conquers two towns in Samnium and all Lucania.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=11