Here is a quote from Institute of Creation Research that I think explains it well.
The Hebrew word for day (
yom) is used some 3,000 times in the Hebrew Bible, and is almost always used to mean an ordinary 24-hour day-night cycle. On the few occasions where it is used to mean an indeterminate period of time, it is always clear from the context that it means something other than a 24-hour day (day of trouble, day of the Lord, day of battle, etc). Whenever it is used with an ordinal (1, 2, 1st, 2nd, etc.), it always means a specific day, an ordinary24-hour day.
The language of Genesis 1 appears to have been crafted so that no reader would mistake the word use for anything other than an ordinary 24-hour day. The light portion is named “day,” and the dark portion is named “night.” Then the “evening and the morning” is Day 1, Day 2, etc. The linguistic formula is repeated for each of the six days, a strange emphasis if the words were to be taken as allegorical or analogous to something other than a day-night cycle.
When God wrote the Ten Commandments with His own finger (certainly the most emphatic action ever taken by God on behalf of His revealed Word), God specifically designated a seventh day to be a “Sabbath” day (rest day) in memory and in honor of the work-six-days, rest-one-day activity of God during the creation week (
Exodus 20:11). In that context, spoken and written by God Himself, the creation week can mean only a regular week of seven days, one of which is set aside as holy.
I realize that this is a debate that has been going on for a long time. I am just saying that as a scientist I have no problem using God's word as the foundation for my studies. I see no conflict between the facts observed and what God says. The conflict arises from interpretation of the facts