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
The Appuleian Laws
Let us recall the objects which Gaius Gracchus pursued, and the means by which he pursued them. His object was to break down the oligarchy within and without. He aimed, on the one hand, to restore the power of the magistrates, which had become completely dependent on the senate, to its original sovereign rights, and to re-convert the senatorial assembly from a governing into a deliberative board; and, on the other hand, to put an end to the aristocratic division of the state into the three classes of the ruling burgesses, the Italian allies, and the subjects, by the gradual equalization of those distinctions which were incompatible with a government not oligarchical. These ideas the three confederates revived in the colonial laws, which Saturninus as tribune of the people had partly introduced already (651), partly now introduced (654).(7)
7. It is not possible to distinguish exactly what belongs to the first and what to the second tribunate of Saturninus; the more especially, as in both he evidently followed out the same Gracchan tendencies. The African agrarian law is definitely placed by the treatise De Viris Ill. 73, 1 in 651; and this date accords with the termination, which had taken place just shortly before, of the Jugurthine war. The second agrarian law belongs beyond doubt to 654. The treason-law and the corn- law have been only conjecturally placed, the former in 651 (p. 442 note), the latter in 654.
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Reference address : https://www.ellopos.net/elpenor/rome/4-06-marius-revolution-drusus-reform.asp?pg=23