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
A real middle class in our sense of the term there was not, as indeed no such class can exist in any fully-developed slave-state; what appears as if it were a good middle class and is so in a certain measure, is composed of those rich men of business and landholders who are so uncultivated or so highly cultivated as to content themselves within the sphere of their activity and to keep aloof from public life. Of the men of business--a class, among whom the numerous freedmen and other upstarts, as a rule, were seized with the giddy fancy of playing the man of quality--there were not very many who showed so much judgment. A model of this sort was the Titus Pomponius Atticus frequently mentioned in the accounts of this period.
He acquired an immense fortune partly from the great estate-farming which he prosecuted in Italy and Epirus, partly from his money-transactions which ramified throughout Italy, Greece, Macedonia, and Asia Minor; but at the same time he continued to be throughout the simple man of business, did not allow himself to be seduced into soliciting office or even into monetary transactions with the state, and, equally remote from the avaricious niggardliness and from the prodigal and burdensome luxury of his time--his table, for instance, was maintained at a daily cost of 100 sesterces (1 pound)-- contented himself with an easy existence appropriating to itself the charms of a country and a city life, the pleasures of intercourse with the best society of Rome and Greece, and all the enjoyments of literature and art.
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Reference address : https://www.ellopos.net/elpenor/rome/5-11-old-republic-new-monarchy.asp?pg=102