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 consul Lucius Opimius offered the usual sacrifice in the porch of the Capitoline temple; one of the attendants assisting at the ceremony, Quintus Antullius, with the holy entrails in his hand, haughtily ordered the "bad citizens" to quit the porch, and seemed as though he would lay hands on Gaius himself; whereupon a zealous Gracchan drew his sword and cut the man down. A fearful tumult arose.
Gracchus vainly sought to address the people and to disclaim the responsibility for the sacrilegious murder; he only furnished his antagonists with a further formal ground of accusation, as, without being aware of it in the confusion, he interrupted a tribune in the act of speaking to the people--an offence, for which an obsolete statute, originating at the time of the old dissensions between the orders,(28) had prescribed the severest penalty.
28. Cf. II. II. Legislation
The consul Lucius Opimius took his measures to put down by force of arms the insurrection for the overthrow of the republican constitution, as they were fond of designating the events of this day. He himself passed the night in the temple of Castor in the Forum; at early dawn the Capitol was filled with Cretan archers, the senate-house and Forum with the men of the government party--the senators and the section of the equites adhering to them--who by order of the consul had all appeared in arms and each attended by two armed slaves.
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Reference address : https://www.ellopos.net/elpenor/rome/4-03-revolution-gaius-gracchus.asp?pg=48