The time denoted by a temporal clause is not always solely contemporaneous, antecedent, or subsequent to that of the principal clause, but may overlap with the time of the principal clause (before and at the same time, at the same time and after, until and after).
ἐπεὶ δὲ ἠσθένει Δα_ρεῖος καὶ ὑπώπτευε τελευτὴν τοῦ βίου, ἐβούλετο τὼ παῖδε παρεῖναι
ὁ δ' ἔν τε τῷ παρόντι πρὸς τὰ μηνύ_ματα ἀπελογεῖτο καὶ ἑτοῖμος ἦν πρὶν ἐκπλεῖν κρί_νεσθαι
a. Conjunctions of antecedent action usually take the aorist, rarely the imperfect except when that tense represents overlapping action, as in
b. A verb of aoristic action is used: in the temporal clause when complete priority, in the main clause when complete subsequence, is to be clearly marked.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].