The word "day" is used three ways in the Bible -- an entire revolution of the earth (i.e. 24 hours); the period of time between sunrise and sunset (i.e. daylight); or some unspecified period of time (i.e. figurative). This would introduce the possibility that the "days" of creation -- those mentioned in the first chapter of Genesis -- might theoretically be any one of these. Which is it?
7 reasons that the first 7 days are 24-hour days rather than 1000-year days or extended periods of time
1. Though "day" is sometimes used of prolonged periods, in all five of Moses' books (the Pentateuch) when "day" is used with a numerical adjective, its meaning is restricted to a literal 24-hour day. Genesis One is no exception.
2. The specific mention of "evening" and "morning" indicates that the Bible is speaking of 24-hour days. These terms would be meaningless if referring to some unknown period of time. Evening and morning refer to a 24-hour day; Bible days began at sunset rather than at midnight as our days (Cf. Gen. 1:5; Exod. 12:18; Lev. 23:32).
3. If the evening were the first half of an extended period of time and the morning the second half, what happened to the plants created on the third day when the evening of the fourth day came? Could they survive 500 years of darkness? The same goes for the animals.
4. Adam was 930 years old when he died (Cf. Gen. 5:5). If Genesis One days were 1000-year periods, Adam would have been over 1000 years old on the second day of his existence, and over 2000 by the time he died -- since he was created on the sixth day. The first seven days were the same kind of days Adam lived all of his life, and by which the days of his life were counted. If each day were a 1000 years, Adam would have been around 334,800,000 years old when he died.
5. Why would it take God 6000 years to create the world? God didn't need 1000-year days -- it's the theistic evolutionists who need that period of time to explain their views. The Genesis account tells of a God who spoke and things came into existence.
6. None said these days were ages before the scientists said the earth was billions of years old. The motive behind lengthening the first seven days is make the Bible conform to science -- to fit the belief that the earth if billions of years old. Many scientific theories oppose God's Word. No proven scientific fact disagrees with the Bible. Should scientists every arrive at the truth, they will find Bible-believers are already there!
7. Man's six work days correspond precisely to God's six work days. Man's day of rest corresponds to God's day of rest. Man's seven-day week is the same as God's seven-day week. See Exodus 20:8-11.
The first seven days are the same kind of days as our 24-hour days -- the first week a week by which all weeks thereafter are patterned. The truth is plain and simple in Genesis, if we have eyes to see and aren't searching for something else.