I am saying that it refers to the time interval between sunrise and sunrise or from dusk to dusk. The ancient Hebrews lacked any means or ability to measure time with any greater precision than yesterday, today, and tomorrow, with the day divided up vaguely with descriptions such as midday, midnight, and half-night.
I suppose you could argue that a day is standard except when it isn't or you would be hard pressed to explain Joshua 10:13 where the sun stood still and the moon stopped, time ran backwards for Hezekiah (2 Kings 20:9-11). I mean, why would you concede that God could and did manipulate time in one instance, but not for creation.
It can be a normal day. There are 4 definitions for Yom in the book of Bereshis (Genesis). A day can mean any length of day that has light and darkness, a solar day, a reference to a previous time (i.e. in my Father’s day), or simply an undesignated length of time (a day of reflection, etc.) Several study bibles will list these options in the footnotes. I firmly believe, as He states, that He is the Creator of all things. Since we are bound by the restraints of this world, it is difficult, if not impossible for us to visualize things from G-d’s perspective, and we cannot contain G-d’s knowledge in our limited mental capacity.