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 state- envoys sent to Syracuse in the year 262 transact business with Dionysius the elder, who ascended the throne eighty-six years afterwards (348). This naive uncritical spirit is especially apparent in the treatment of Roman chronology. Since according to the Roman reckoning--the outlines of which were probably fixed in the previous epoch--the foundation of Rome took place 240 years before the consecration of the Capitoline temple(63) and 360 years before the burning of the city by the Gauls,(64) and the latter event, which is mentioned also in Greek historical works, fell according to these in the year of the Athenian archon Pyrgion 388 B. C. Ol. 98, i, the building of Rome accordingly fell on Ol. 8, i.
63. Cf. II. IX. Roman Early History of Rome
64. Cf. II. IX. Registers of Magistrates
This was, according to the chronology of Eratosthenes which was already recognized as canonical, the year 436 after the fall of Troy; nevertheless the common story retained as the founder of Rome the grandson of the Trojan Aeneas.
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Reference address : https://www.ellopos.net/elpenor/rome/3-14-literature-art.asp?pg=97