The preposition intrā takes an accusative. It is likely derived from the adjectival feminine adjective singular of inter, intra, intrum (intrā) — an archaic adjective which also produced the corresponding preposition inter.
Intra + accusative is primarily used with a single class of nouns, and denotes a space ‘within which.’
intrā moenia, within the walls
intrā me deus est, the Lord is within me
intrā iactum telī, within a javelin’s throw (denoting distance)
Intra + accusative of time is one ways of denoting the time within something occurred.
intrā quattuor annōs, within four years
intrā lucem, before the day was done
intrā diēs paucōs, within few days (before a few days had passed)
intrā morae breve tempus, without a moment’s delay
It can also mean less than a given duration or quantity.
intrā centum fūnera fēcit, he inflicted fewer than 100 casualties
intrā trēs diēs abiit, he left before three days had passed (compare above)