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 Restored Aristocracy
So it accordingly happened. After the decease of Gaius Gracchus without heirs, the government of the senate as it were spontaneously resumed its place; and this was the more natural, that it had not been, in the strict sense, formally abolished by the tribune, but had merely been reduced to a practical nullity by his exceptional proceedings.
Yet we should greatly err, if we should discern in this restoration nothing further than a relapse of the state-machine into the old track which had been trodden and worn for centuries. Restoration is always revolution; but in this case it was not so much the old government as the old governor that was restored. The oligarchy made its appearance newly equipped in the armour of the -tyrannis- which had been overthrown.
As the senate had beaten Gracchus from the field with his own weapons, so it continued in the most essential points to govern with the constitution of the Gracchi; though certainly with the ulterior idea, if not of setting it aside entirely, at any rate of thoroughly purging it in due time from the elements really hostile to the ruling aristocracy.
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Reference address : https://www.ellopos.net/elpenor/rome/4-04-rule-restoration.asp?pg=2