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
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.