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
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.