The Torah still translates to "on the first day G-d did thus and such, on the second day..." etcetera. The word day meant the same thing to humans then as it did now.

The Jewish belief is that G-d used the word "day" so that humans at the time would be able to understand what he meant. It's a simplification. If G-d said "well, let's see, about 40 billion years ago I made your little rock of a planet" the humans at the time would have no clue what he was talking about.