The Ablative of time within which is used to express the time within which something happens.
It is usually expressed without a latin preposition
EX: quīntō diē - within four days (Dickinson.edu).
Tribus diebus discedeamus- We shall leave within three days (The Latin Library).
Paucïs horïs id faciet- he will do it within a few hours (abacus.bates).