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 cavalry on the other hand--with the exception of a noble guard, more respectable than militarily important, formed from the young aristocracy of Rome, and of the Apulian slave-herdsmen whom Pompeius had mounted (25)--consisted exclusively of the contingents of the subjects and clients of Rome.
25. Cf. V. X. The Pompeian Army
The flower of it consisted of the Celts, partly from the garrison of Alexandria,(26) partly the contingents of king Deiotarus who in spite of his great age had appeared in person at the head of his troops, and of the other Galatian dynasts.
26. Cf. V. IV. And Brought Back by Gabinius
With them were associated the excellent Thracian horsemen, who were partly brought up by their princes Sadala and Rhascuporis, partly enlisted by Pompeius in the Macedonian province; the Cappadocian cavalry; the mounted archers sent by Antiochus king of Commagene; the contingents of the Armenians from the west side of the Euphrates under Taxiles, and from the other side under Megabates, and the Numidian bands sent by king Juba--the whole body amounted to 7000 horsemen.
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Reference address : https://www.ellopos.net/elpenor/rome/5-10-brundisium-pharsalus-thapsus.asp?pg=63