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
Term of Office
As regards the termination of their tenure of office, the earlier -interregnum- of five days furnished a legal precedent. The ordinary presidents of the community were bound not to remain in office longer than a year reckoned from the day of their entering on their functions;(4) and they ceased -de jure- to be magistrates upon the expiry of the year, just as the interrex on the expiry of the five days.
4. The day of entering on office did not coincide with the beginning of the year (1st March), and was not at all fixed. The day of retiring was regulated by it, except when a consul was elected expressly in room of one who had dropped out (-consul suffectus-); in which case the substitute succeeded to the rights and consequently to the term of him whom he replaced.
But these supplementary consuls in the earlier period only occurred when merely one of the consuls had dropped out: pairs of supplementary consuls are not found until the later ages of the republic. Ordinarily, therefore, the official year of a consul consisted of unequal portions of two civil years.
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=9