A fuller entry on the same topic at alpha 802. For an example of the use of the phrase in this sense, without a word explicitly denoting 'time': [Lucian], Asinus 37. The adjective a)karh=s, connected with the verb kai/rw literally means 'too short to be cut'; hence e)n a)karei= xro/nw| ('in an instant [i.e. indivisible quantity] of time) in the quotation given, and elsewhere e.g. in the mss. of Aristophanes, Plutus [Wealth] 244. Also e)n a. xro/nou or tou= xro/nou: Alciphron 3.56, Lucian, Timon 3 and 23. The dative a)karei= is used as an adverb by Plutarch, Sertorius 16.
[1] = Synagoge epsilon372, Photius epsilon802; cf. Hesychius epsilon2276, and contrast Hesychius epsilon2624, with the gloss e)n r(iph=|, 'in a rush'.
[2] Once [Polybius] fr. 120 Büttner-Wobst, but nowadays attributed to book 30 of Diodorus Siculus: see at alpha 3680, and cf. delta 584, epsilon 254.