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
Political Position of Marius
Marius stood aloof from the parties not much less than from society. The measures which he carried in his tribunate of the people (635)--a better control over the delivery of the voting-tablets with a view to do away with the scandalous frauds that were therein practised, and the prevention of extravagant proposals for largesses to the people(1)--do not bear the stamp of a party, least of all that of the democratic, but merely show that he hated what was unjust and irrational; and how could a man like this, a farmer by birth and a soldier by inclination, have been from the first a revolutionist?
1. Cf. IV. IV. The Domain Question under the Restoration
The hostile attacks of the aristocracy had no doubt driven him subsequently into the camp of the opponents of the government; and there he speedily found himself elevated in the first instance to be general of the opposition, and destined perhaps for still higher things hereafter. But this was far more the effect of the stringent force of circumstances and of the general need which the opposition had for a chief, than his own work; he had at any rate since his departure for Africa in 647-8 hardly tarried, in passing, for a brief period in the capital. It was not till the latter half of 653 that he returned to Rome, victor over the Teutones as over the Cimbri, to celebrate his postponed triumph now with double honours--decidedly the first man in Rome, and yet at the same time a novice in politics.
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Reference address : https://www.ellopos.net/elpenor/rome/4-06-marius-revolution-drusus-reform.asp?pg=4