The question of evening and morning is for another occasion; our present subject is the meaning of “day”.
In “Alice in Wonderland” the Mad Hatter more or less says that words mean whatever we want them to. More precisely, Einstein says that the speed at which time passes is not the same in all circumstances.
Nachmanides and others argue that when the Bible says “day” it means the same as the days of hours and minutes with which human beings are familiar.
Not all the commentators agree, and many point to Psalm 90:4 which declares, ”A thousand years in Your sight are like yesterday”, indicating that God’s days are not necessarily 24 hours but are vast expanses of time.
In fact there is no such thing as God’s days. God is outside the time system and, unlike human beings, is not governed by the passing of time.
When the Torah speaks of the days of creation it is using human language, implying that if God did actually have days they would be eras.
When we refer to evening and morning being one day, we are not measuring time according to any particular cultural usage.