"For in six days the Lord made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the Lord blessed the sabbath day, and hallowed it." (Exodus 20:11)
God's word is omnipotent, and He could just as well have created an entire universe, fully populated and functioning, in an instant of time. Instead, He chose to do it in six days, with a seventh day to be set aside as a day of rest and remembrance of His completed, "very good," creation. Since that time, it has been the universal practice among monotheists--those who believe in one Creator God--to measure time in seven-day weeks, with one of those days observed as a day of rest and worship of the Creator.
This divine assertion was inscribed with "the finger of God" on a table of stone (Exodus 31:18), clearly settling, once and for all, the ancient question of the age of the cosmos, at least for those who really believe in the inerrant perspicuity and authority of the Holy Scriptures. Not only did the Lord precisely equate the six days of man's work week with the six days of His own work week, He then pronounced it all "very good" and "sanctified" the seventh day (Genesis 1:31, 2:3). This would have been an unthinkable thing for Him to say if there were, at that time, a great mile-deep graveyard, consisting of the fossil remains of dead animals from the so-called geological ages, extending all around the globe. These fossils must all be dated as post-Eden, after human sin and God's curse brought death into the world (Romans 5:12).
Today, those who believe in God and creation should certainly continue to remember Him by observing every seventh day as a day of rest and worship, in honor of their Creator, who has now also become their Redeemer and who will soon come again to reign as eternal King. HMM