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 — dativeon the third day
  3. kind of time — genitiveby 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.