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
Roman Historical Composition
Along with the rudiments of the Roman drama, the rudiments of Roman historical composition belong to this period; both as regards the contemporary recording of remarkable events, and as regards the conventional settlement of the early history of the Roman community.
Registers of Magistrates
The writing of contemporary history was associated with the register of the magistrates. The register reaching farthest back, which was accessible to the later Roman inquirers and is still indirectly accessible to us, seems to have been derived from the archives of the temple of the Capitoline Jupiter; for it records the names of the annual presidents of the community onward from the consul Marcus Horatius, who consecrated that temple on the 13th Sept. in his year of office, and it also notices the vow which was made on occasion of a severe pestilence under the consuls Publius Servilius and Lucius Aebutius (according to the reckoning now current, 291), that thenceforward a nail should be driven every hundredth year into the wall of the Capitoline temple.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=7