And if a day is the rotation of the Earth WRT the sun, then what is a day before there is a sun or, strictly speaking, the Earth*?
Why would the first day close with "And there was evening and there was morning, the first day"?
* Gen 1:2 -- And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
if it is not a 24 hour period, like every other day is, then it’s meaning would be...meaningless
So, since the word YOM is the word used, and YOM is a 24 hour day...
Always trust the Bible, and doubt those who teach you to trust something else