Here's a few interesting snippits from a publication:
What about the length of the creative days? Were they literally 24 hours long? Some claim that because Moses—the writer of Genesis—later referred to the day that followed the six creative days as a model for the weekly Sabbath, each of the creative days must be literally 24 hours long. (Exodus 20:11) Does the wording of Genesis support this conclusion?
No, it does not. The fact is that the Hebrew word translated “day” can mean various lengths of time, not just a 24-hour period. For example, when summarizing God’s creative work, Moses refers to all six creative days as one day. (Genesis 2:4) In addition, on the first creative day, “God began calling the light Day, but the darkness he called Night.” (Genesis 1:5) Here, only a portion of a 24-hour period is defined by the term “day.” Certainly, there is no basis in Scripture for arbitrarily stating that each creative day was 24 hours long.
How long, then, were the creative days? The wording of Genesis chapters*1 and 2 indicates that considerable lengths of time were involved.
Moses wrote his account in Hebrew, and he wrote it from the perspective of a person standing on the surface of the earth. These two facts, combined with the knowledge that the universe existed before the beginning of the creative periods, or “days,” help to defuse much of the controversy surrounding the creation account. How so?
A careful consideration of the Genesis account reveals that events starting during one “day” continued into one or more of the following days. For example, before the first creative “day” started, light from the already existing sun was somehow prevented from reaching the earth’s surface, possibly by thick clouds. (Job 38:9) During the first “day,” this barrier began to clear, allowing diffused light to penetrate the atmosphere.
On the second “day,” the atmosphere evidently continued to clear, creating a space between the thick clouds above and the ocean below. On the fourth “day,” the atmosphere had gradually cleared to such an extent that the sun and the moon were made to appear “in the expanse of the heavens.” (Genesis 1:14-16) In other words, from the perspective of a person on earth, the sun and moon began to be discernible. These events happened gradually.
The Genesis account also relates that as the atmosphere continued to clear, flying creatures—including insects and membrane-winged creatures—started to appear on the fifth “day.” However, the Bible indicates that during the sixth “day,” God was still in the process of “forming from the ground every wild beast of the field and every flying creature of the heavens.”—Genesis 2:19.
Clearly, the Bible’s language makes room for the possibility of some major events during each “day,” or creative period, to have occurred gradually rather than instantly, perhaps some of them even lasting into the following creative “days.”
