Using emperors' reigns to date events/people

[prr]

The church father Jerome says in his Illustrious Men 17, "Afterwards during the reign of Marcus Antoninus and Lucius Aurelius Commodus in the fourth persecution after Nero, in the presence of the proconsul holding court at Smyrna and all the people crying out against him in the Amphitheater, he was burned. He wrote a very valuable Epistle to the Philippians which is read to the present day in the meetings in Asia." Jerome was discussing the martyrdom of Polycarp.

 

Now when he says "during the reign of Marcus Antoninus and L. Aurelius Commodus," does he mean (1) at some point during the reigns of either of the emperors, i.e., at any time from 161-192, or (2) specifically, during the time when both were emperor (apparently Commodus was co-emperor from 177 till the death of his father in 180)?

 

The Latin from the text of Jerome reads: regnante M. Antonino, et L. Aurelio Commodo.

 

The use of the gerund regnante, lit. "reigning" suggests that both were ruling, but this is conjecture on my part.

[caldrail]

It refers to the period of joint rule otherwise the reference would have been different. The Romans used such means to date events as a matter of course - in earlier times the event was listed as taking place in the consulship of the two Romans concerned.

[Maladict]

The persecution (assuming it refers to Lyons) happened in 177, so that would fit nicely with joint rule. Note that Jerome wrote circa AD 400 and is therefore not a primary source.