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
Time and Occasion of the Reform
When and how this new military organization of the Roman community came into existence, can only be conjectured. It presupposes the existence of the four regions; in other words, the Servian wall must have been erected before the reform took place. But the territory of the city must also have considerably exceeded its original limits, when it could furnish 8000 holders of full hides and as many who held lesser portions, or sons of such holders. We are not acquainted with the superficial extent of the normal Roman farm; but it is not possible to estimate it as under twenty -jugera-.(12)
12. Even about 480, allotments of land of seven -jugera- appeared to those that received them small (Val. Max. iii. 3, 5; Colum. i, praef. 14; i. 3, ii; Plin. H. N. xviii. 3, 18: fourteen -jugera-, Victor, 33; Plutarch, Apophth. Reg. et Imp. p. 235 Dubner, in accordance with which Plutarch, Crass. 2, is to be corrected).
A comparison of the Germanic proportions gives the same result. The -jugerum- and the -morgen- [nearly 5/8 of an English acre], both originally measures rather of labour than of surface, may be looked upon as originally identical. As the German hide consisted ordinarily of 30, but not unfrequently of 20 or 40 -morgen-, and the homestead frequently, at least among the Anglo-Saxons, amounted to a tenth of the hide, it will appear, taking into account the diversity of climate and the size of the Roman -heredium- of 2 -jugera-, that the hypothesis of a Roman hide of 20 -jugera- is not unsuitable to the circumstances of the case. It is to be regretted certainly that on this very point tradition leaves us without precise information.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=21