The most simple way to express time in Greek is to put the adjective, if there is one, and the noun that specify a time element in one of three cases to indicate the particular nuance of time being expressed

1. time how long (extent) — accusative — for three days
2. time at which, time when — dative — on the third day
3. kind of time — genitive — by day

Dative and genitive indication of time are easy to remember, for their use is close to what we see in other uses of the dative and genitive. Dative represent a point or position, without a sense of extent or motion (see Prepositions and Cases), thus for time, it expresses "time when." Genitive indicates the kind of material (such as a coat of leather), thus for time, it reflects the kind of time. All that is left is the accusative case, indicating time how long.