"And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day." (Genesis 1:5)
Many people today, professing to believe the Bible, have compromised with the evolutionary philosophy which dominates our society by accepting its framework of geological ages. This system interprets the rocks and fossils in terms of a supposed 4.6 billion-year history of the earth and life culminating in the evolution of early humans perhaps a million years ago. In order to justify this compromise, they usually say that the "days" of creation really correspond to the geological ages, arguing that the Hebrew word for "day" (yom) does not have to mean a literal solar day.
Oh, yes, it does—at least in Genesis chapter one! God, knowing that the pagan philosophers of antiquity would soon try to distort His record of creation into long ages of pantheistic evolution (as in the Babylonian, Egyptian, Greek, and other such ancient cosmogonies), was careful to define His terms! "God called the light Day," and that was the first day with its evening and morning. All subsequent days have followed the same pattern—a period of darkness (night), then a period of light (day).
One may quibble about the exact length of the day if he insists (e.g., equatorial days versus polar days), but there is no way this definition can accommodate a geological age. This is the very first reference to "day" (or yom) in the Bible, and this is given as an actual statement of the meaning of the word.
This ought to settle the question for anyone who really believes the Bible. One may decide to believe the evolutionary geologists if he wishes, instead of God, but he should at least let God speak for Himself. God says the days of creation were literal days, not ages. "In six days the LORD made heaven and earth" (Exodus 31:17). HMM