Creationists don't appear to have any problem assuming that a "day" in the past is the same 24 hours we have now. That 24 hours is always measured by the rate of change of some process. You appear to want to restrict the luxury of assuming constants to being your exclusive domain.
Actually, rather than assuming, I trust that God and his words are true. Since he wrote the DNA for all living things why should anyone assume different meanings for his very plain and straightforward words ~ there was evening and there was morning ~ the same way we measure days today.
Furthermore, how do evolutionists explain the 7 day week?
Month = approx lunar cycle and year = solar cycle but what about that pesky week?