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
On the very day after he had succeeded in cutting off the Pompeians from the Ebro, the soldiers of the two armies had begun to fraternize and to negotiate respecting surrender; indeed the terms asked by the Pompeians, especially as to the sparing of their officers, had been already conceded by Caesar, when Petreius with his escort consisting of slaves and Spaniards came upon the negotiators and caused the Caesarians, on whom he could lay hands, to be put to death. Caesar nevertheless sent the Pompeians who had come to his camp back unharmed, and persevered in seeking a peaceful solution.
Ilerda, where the Pompeians had still a garrison and considerable magazines, became now the point which they sought to reach; but with the hostile army in front and the Sicoris between them and the fortress, they marched without coming nearer to their object. Their cavalry became gradually so afraid that the infantry had to take them into the centre and legions had to be set as the rearguard; the procuring of water and forage became more and more difficult; they had already to kill the beasts of burden, because they could no longer feed them. At length the wandering army found itself formally inclosed, with the Sicoris in its rear and the enemy's force in front, which drew rampart and trench around it. It attempted to cross the river, but Caesar's German horsemen and light infantry anticipated it in the occupation of the opposite bank.
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Reference address : https://www.ellopos.net/elpenor/rome/5-10-brundisium-pharsalus-thapsus.asp?pg=43