The aorist is commonly used with definite numbers. The imperfect is, however, often employed when an action is represented as interrupted or as proceeding from one stage to another. Thus,
ἐνταῦθα ἔμεινε Κῦρος ἡμέρα_ς τριά_κοντα
τέτταρας μῆνας ὅλους ἐσῴζοντο οἱ Φωκεῖς τοὺς ὕστερον, ἡ δὲ τούτου ψευδολογία_ μετὰ ταῦθ' ὕστερον αὐτοὺς ἀπώλεσεν
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].