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 "people" accordingly ratified the Livian laws as readily as it had before ratified the Sempronian. It then, as usual, repaid its latest, by inflicting a gentle blow on its earlier, benefactor, declining to re-elect him when he stood for the third time as a candidate for the tribunate for the year 633; on which occasion, however, there are alleged to have been unjust proceedings on the part of the tribune presiding at the election, who had been formerly offended by Gracchus.
Thus the foundation of his despotism gave way beneath him. A second blow was inflicted on him by the consular elections, which not only proved in a general sense adverse to the democracy, but which placed at the head of the state Lucius Opimius, who as praetor in 629 had conquered Fregellae, one of the most decided and least scrupulous chiefs of the strict aristocratic party, and a man firmly resolved to get rid of their dangerous antagonist at the earliest opportunity.
Do you see any typos or other mistakes? Please let us know and correct them
Reference address : https://www.ellopos.net/elpenor/rome/4-03-revolution-gaius-gracchus.asp?pg=46