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
Perseus Is Driven Back to Pydna - Battle of Pydna - Perseus Taken Prisoner
As soon as the new general arrived in the camp at Heracleum, he gave orders for the ill-guarded pass at Pythium to be surprised by Publius Nasica, while skirmishes between the outposts in the channel of the river Elpius occupied the attention of the Macedonians; the enemy was thus turned, and was obliged to retreat to Pydna.
There on the Roman 4th of September, 586, or on the 22nd of June of the Julian calendar --an eclipse of the moon, which a scientific Roman officer announced beforehand to the army that it might not be regarded as a bad omen, affords in this case the means of determining the date--the outposts accidentally fell into conflict as they were watering their horses after midday; and both sides determined at once to give the battle, which it was originally intended to postpone till the following day.
Passing through the ranks in person, without helmet or shield, the grey-headed Roman general arranged his men. Scarce were they in position, when the formidable phalanx assailed them; the general himself, who had witnessed many a hard fight, afterwards acknowledged that he had trembled.
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Reference address : https://www.ellopos.net/elpenor/rome/3-10-third-macedonian-war.asp?pg=31