"Time", as we understand it, was created when God spoke the universe into being.
The concept of a "day" [yom] was initiated on the first day, with light representing day, darkness night and evening and morning a complete day. Grammatically, any day [yom] modified by a number or similar structure (evening and morning) represents a literal day. Since God had established light and darkness as day and night and morning and evening on the first day, it seems somewhat simple that the measurement has been consistent since that first day. IMO, the sun and moon simply give us references to the cause as of the third day, but we have nothing else to really grasp as a source of light on the first. Was it just "light"? Was it God's glory? One day maybe we'll find out.
Having said that, the cataclysm that caused the flood very well could have been of such magnitude to have altered the rotation of the earth, so I would not be dogmatic about a 24 hr day. Depending on which hypothesis one considers most likely, it might have been slightly longer or shorter... but not millions of years longer or shorter. :)