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
Indeed, if at any time by way of exception a patrician disinclined to the rule of the nobility was called to the government, his official authority was paralyzed partly by the priestly colleges, which were pervaded by an intense aristocratic spirit, partly by his colleague, and was easily suspended by the dictatorship; and, what was of still more moment, he wanted the first element of political power, time.
The president of a commonwealth, whatever plenary authority may be conceded to him, will never gain possession of political power, if he does not continue for some considerable time at the head of affairs; for a necessary condition of every dominion is duration. Consequently the senate appointed for life inevitably acquired--and that by virtue chiefly of its title to advise the magistrate in all points, so that we speak not of the narrower patrician, but of the enlarged patricio-plebeian, senate--so great an influence as contrasted with the annual rulers, that their legal relations became precisely inverted; the senate substantially assumed to itself the powers of government, and the former ruler sank into a president acting as its chairman and executing its decrees.
In the case of every proposal to be submitted to the community for acceptance or rejection the practice of previously consulting the whole senate and obtaining its approval, while not constitutionally necessary, was consecrated by use and wont; and it was not lightly or willingly departed from.
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=34