That really just seems like a guess, because it isn’t explicit. But the important thing, is that it doesn’t matter at all as a matter of salvation. If you think the earth is a few thousand years, and I think it a few billion it doesn’t change the fundamental of Christ on the cross for our sins.
No young earther, or old earther is condemned for that belief. That really is some magic in the plan if you think about it. So study away and enjoy yourself. Nothing bad can come of that.
Amen, brother!