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Hebrew for "day"
The Hebrew word for "day" in Genesis 1 is "yome" (Strong's 03117). It can mean a 24-hour day or the daylight portion of it (day as distinct from the night).
Without exception, in the Hebrew Old Testament, the word "yome" is never used to refer to a long period of time, as in thousands or millions of years.
In Hebrew, should the word "yome" be used in an indefinite sense, it will be clearly indicated by the context that the literal meaning is not intended.
First-time use not symbolic
Some people say that the word "day" in Genesis is used symbolically.
This is impossible as a word cannot be symbolic the first time it is used. It can only be used symbolically if it first has a literal meaning.
For example, we are told that Jesus is the "bread of life". We know what this means because we understand the literal meaning of "bread", and are able to apply it symbolically to Jesus. The word "bread" cannot be used in this sense unless it first has a literal meaning.
Likewise, the word "day" cannot be used symbolically the first time it appears in Genesis, as this is where God introduced the word "day" and defined it as He created it.
Some might argue that this point is flawed because Job is an earlier book, in the sense that Job lived before the time of Moses. But this is to imply that the Holy Spirit was outdated when He inspired Moses to write Genesis, and that He made a mistake when He put Genesis as the first book of the Bible.
The Bible itself defines "day"
Many Christians forget that the Holy Spirit himself has defined the word "day" the first time it appears in the Bible. A basic rule of thumb in Bible study is to let the Bible interpret the Bible.
Genesis 1:5
5 God called the light "day," and the darkness he called "night." And there was evening, and there was morning-the first day.
The first time the word "day" is used, it is defined as "the light" to distinguish it from "the darkness" called "night".
The phrase, "and there was evening, and there was morning", is used for each of the other five days of creation. This shows that there was a clearly established cycle of days and nights (periods of light and periods of darkness).
Incidentally, those who argue that the word "day" in the above verse means millions of years must answer the question, "What is a night?"
Daylight without the sun?
But how could there be day and night when the sun was not created yet, until day four?
The word for "light" in Genesis 1:3 means the "substance" of light that was created. Then, on day four in Genesis 1:14-19, we are told of the creation of the sun, which was to be the source of light henceforth.
The sun was created to rule the day that already existed. The day merely had a new light source.
Perhaps God deliberately left the creation of the sun to the fourth day to show that He is the light, the source of life and the sustainer of life, because He knew that man would one day worship the sun as the source of life.
Problems with taking "day" to mean millions of years
Our seven-day week
Exodus 20:9 tells us that we are to work for six days and rest for one. This is why we have a seven-day week.
Exodus 20:9
9 Six days you shall labor and do all your work,
The reason for this is found in verse 11:
Exodus 20:11
11 For in six days the Lord made the heavens and the earth, the sea, and all that is in them, but he rested on the seventh day. Therefore the Lord blessed the Sabbath day and made it holy.
This is a direct reference to God's creation week in Genesis 1. To be consistent, whatever is used as the meaning of the word "day" in Genesis 1 must also be used here.
So, if we take "day" in Genesis 1 to mean millions of years, then we should do the same for Exodus 20:11, which would make nonsense of our seven-day week. We don't work for six million years and then rest for 1 million years!
What are "years" and "seasons" then?
Genesis 1:14
14 And God said, "Let there be lights in the expanse of the sky to separate the day from the night, and let them serve as signs to mark seasons and days and years,
If the word "day" here is not to be taken literally, then, to be consistent in our interpretation, neither should the words "seasons", "days" and "years". What do they mean then?
Likewise, we are told in Genesis 1:26-31 that God made Adam on the sixth day. We know that Adam lived through the rest of the sixth day and through the seventh day. Genesis 5:5 says that he died when he was 930 years old.
If we take "day" in Genesis 1 to mean millions of years, how do we understand Adam's lifespan of "930 years"? What is a "year"? Or, for that matter, a "night", a "week", a "month"?
Covenant with day and night
Jeremiah 33:25,26
25 This is what the Lord says: 'If I have not established my covenant with day and night and the fixed laws of heaven and earth,
26 then I will reject the descendants of Jacob and David my servant and will not choose one of his sons to rule over the descendants of Abraham, Isaac and Jacob. For I will restore their fortunes and have compassion on them.' "
God's "covenant with day and night" began in Genesis 1. There is no clear origin and definition for day and night in the Bible other than Genesis 1. Therefore, this must be where the covenant began.
However, this covenant would make no sense and be on shaky ground if "day" is not taken literally in Genesis 1. And, again, what would "night" mean?
A day as a thousand years?
2 Peter 3:8
8 But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day.
Psalm 90:4
4 For a thousand years in your sight are like a day that has just gone by, or like a watch in the night.
The two verses are used by many to teach that the days in Genesis must each be a thousand years long.
But the verses are not saying that God defines a "day" as "a thousand years". That would contradict His original definition in Genesis 1:5. Also, note that the word "like" is used.
In both cases, the truth being presented is that God is neither limited by natural processes nor by time. The Creator of time is not bound by time.
Also, neither verse refers to the days of creation in Genesis. In 2 Peter 3, the context is Christ's second coming. In Psalm 90, the context is Israel's rebellion in the wilderness and the mortality of man.
The verses also indicate that God, not bounded by time, can do in a very short time what men or nature would require a very long time to accomplish, if they could succeed at all.
Interestingly, evolutionists say that the chance, random processes of nature required millions of years to produce living things and man. Many Christians have accepted this by saying that God took millions of years to create, which is the very opposite of what 2 Peter 3:8 and Psalm 90:4 are saying.