Expressing time duration or time when and within

Question

I'm studying Oxford Latin Course. The book said that duration of time takes the accusative and time when and within take the ablative case. I can understand that, but does duration of time and time within take cardinal and time when take ordinal? Like, tres horas = “for three hours,” decem diebus = “within ten days”?

Answer 1

That is exactly right:

Dominus creavit caelum et terram sex diebus. Septimo die requievit ab operibus suis.

The Lord created heaven and earth in six days. (i.e., within, over the course of six days) On the seventh day he rested from his work.

Answer 2

decem diebus is "time within which" yet decem is a cardinal number. So I think within which can take either ordinal or cardinal numbers with it.