I have been reading the final volume of the magnificent commentary on Ammianus Marcellinus by Den Boeft, Den Hengst, Drijvers and Teitler (to be duly celebrated in Leiden next month). They do full justice to the magnificent climax of Ammianus’ work.
Valens’ obituary opens with his age and length of reign (31.14.1; I indicate clausulae with a hasta):
Perit autem hoc exitu Valens/ quinquagesimo anno contiguus,/ cum per annos quattuor imperasset et decem/ parvo minus. Cuius bona multis cognita dicemus et vitia./
And with such a death perished Valens, close to his fiftieth year, when he had held supreme command for fourteen years, a little less. We shall tell of his good qualities and his faults, which are widely known.
As the commentators note, the phrase parvo minus, ‘although easily understood, is without parallel.’ As they also note subsequently, it is incorrect. When he died on 9 August 378 Valens had been emperor for fourteen years and four months, having been appointed on 30 March 364. Thirdly, we can add that it is also fails to produce one of the regular clausulae that can be found with nearly 100% regularity in Ammianus’ text – even though had indulged in a hyperbaton to produce one immediately beforehand, cum per annos quattuor imperasset et decem, ‘when he had reigned for four years and ten’. (And if anybody suggests that pár-u-o minus might work, there is no good reason to think that v / consonantal u was ever vocalised in Ammianus’ usage, except after q- or in words like suadeo).
All of this would suggest that the two words do not belong, and a reason for them to have made their way into the text is not hard to find: they are a gloss explaining the word contiguus, ‘close to, touching on’, in the line above, which is only very rarely used temporally. The gloss may have been either marginal or interlinear in an ancestor of the Vatican manuscript, our only authoritative source for this book, but the distance is almost exactly one line, which averaged 42 characters in the immediate exemplar of the Vaticanus.
Addendum: It would perhaps only be fair to add the case in favour of retaining the words parvo minus or something like them. The positive argument would be that on the death of Valens’ brother, before his obituary, a similar formula is found, saying correctly that he died aetatis quinquagesimo anno et quinto, imperii minus centum dies secundo et decimo (30.6. 6).