Makes no difference if you think it means “period of time”. Because if you look at the order of creation, and try to stick a long “period of time” in place of a “day”, most of the creation of one day would have died out before the creation of the next “day”, especially before the actual day and night were created.
I don’t have a strong opinion on the age of the earth, but I am inclined to believe that at some point, God did a 6-day creation. It makes little sense to take millions of years when you can do it all in 6 days.
Sorry, no! Yom is defined by context. In this case, the use of evening and morning clarifies the rotational day.