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 Popular Party
Marius therefore discharged the army after his triumph in accordance with the existing regulation, and entered on the course traced out by Gaius Gracchus for procuring to himself supremacy in the state by undertaking its constitutional magistracies. In this enterprise he found himself dependent for support on what was called the popular party, and sought his allies in its leaders for the time being all the more, that the victorious general by no means possessed the gifts and experiences requisite for the command of the streets. Thus the democratic party after long insignificance suddenly regained political importance. It had, in the long interval from Gaius Gracchus to Marius, materially deteriorated.
Perhaps the dissatisfaction with the senatorial government was not now less than it was then; but several of the hopes, which had brought to the Gracchi their most faithful adherents, had in the meanwhile been recognized as illusory, and there had sprung up in many minds a misgiving that this Gracchan agitation tended towards an issue whither a very large portion of the discontented were by no means willing to follow it. In fact, amidst the chase and turmoil of twenty years there had been rubbed off and worn away very much of the fresh enthusiasm, the steadfast faith, the moral purity of effort, which mark the early stages of revolutions.
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Reference address : https://www.ellopos.net/elpenor/rome/4-06-marius-revolution-drusus-reform.asp?pg=18