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
Subsequently it was the state officials who were learned in measuring and in writing, or in other words, the pontifices, that kept an official record of the names of the annual chief magistrates, and thus combined an annual, with the earlier monthly, calendar. Both these calendars were afterwards comprehended under the name of Fasti--which strictly belonged only to the list of court-days.
This arrangement was probably adopted not long after the abolition of the monarchy; for in fact an official record of the annual magistrates was of urgent practical necessity for the purpose of authenticating the order of succession of official documents. But, if there was an official register of the consuls so old, it probably perished in the Gallic conflagration (364); and the list of the pontifical college was subsequently completed from the Capitoline register which was not affected by that catastrophe, so far as this latter reached back.
That the list of presidents which we now have --although in collateral matters, and especially in genealogical statements, it has been supplemented at pleasure from the family pedigrees of the nobility--is in substance based from the beginning on contemporary and credible records, admits of no doubt. But it reproduces the calendar years only imperfectly and approximately: for the consuls did not enter on office with the new year, or even on a definite day fixed once for all; on the contrary from various causes the day of entering on office was fluctuating, and the -interregna- that frequently occurred between two consulates were entirely omitted in the reckoning by official years.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=8