PUL's Cork Irish Bible, the beginning of Joel 3:1

"Óir féach, in sna laethibh sin agus san aimsir sin......

Why do we use the dative form of lá here, laethibh? I thought we just use the dative for nouns acting as an indirect object being acted upon.

Thank you.

Dative is used after dative prepositions and that is almost every simple preposition: ag, ar, as, chuig, de, do, faoi, go, i, le, ó, os, roimh, thar, trí, um