Some people tend to think that the word day used in the book of Genesis must not be literal and that it must actually mean millions of years to coincide with the timeline that modern day scientists tend to believe.
However, the Bible is pretty clear with its reference to "day." The Hebrew word used here for "day" is "yom." We find in the book of Genesis 1:5, "God called the light day, and the darkness he called night, there was evening and there was morning, the first day."
It is clear in scripture that a day simply means a day (evening and morning). We know that it took God six days to create everything. When you read Genesis chapter 1 you will find that after each instance of creation, the Lord says "So the evening and the morning were the second day... third day.. fourth day etc."
If the word day is figurative and not literal then the 4th commandment would not make any sense. We find in Exodus chapter 20, "In six 'days' the Lord made the heavens and the earth." We also read, "Remember the Sabbath and to sanctify it. Six days you shall work and accomplish all your work, but the seventh day is Sabbath to God. You shall not do any work." For God rested on the seventh day (establishing a week.) God establishes His structure from the beginning...He separated the light from the darkness and called it a day.
God is God, we trust Him at His Word...