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
Accordingly the gates of Roman citizenship, which had so long remained closed against entreaty, now suddenly opened when the sword knocked at them; yet even now not fully and wholly, but in a manner reluctant and annoying even for those admitted. A law carried by the consul Lucius Caesar(16) conferred the Roman franchise on the burgesses of all those communities of Italian allies which had not up to that time openly declared against Rome; a second, emanating from the tribunes of the people Marcus Plautius Silvanus and Gaius Papirius Carbo, laid down for every man who had citizenship and domicile in Italy a term of two months, within which he was to be allowed to acquire the Roman franchise by presenting himself before a Roman magistrate.
16. The Julian law must have been passed in the last months of 664, for during the good season of the year Caesar was in the field; the Plautian was probably passed, as was ordinarily the rule with tribunician proposals, immediately after the tribunes entered on office, consequently in Dec. 664 or Jan. 665.
But these new burgesses were to be restricted as to the right of voting in a way similar to the freedmen, inasmuch as they could only be enrolled in eight, as the freedmen only in four, of the thirty-five tribes; whether the restriction was personal or, as it would seem, hereditary, cannot be determined with certainty.
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Reference address : https://www.ellopos.net/elpenor/rome/4-07-sulpician-revolution.asp?pg=38