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.