The Criterion - 25:59
He created the heavens and the earth and all that is between them in six days and then established His authority. He is the Most Merciful; ask those who know Him well.
Six days is unit time; it refers to the time periods of creation:
Explained in Detail/41:9
"Say, "Do you disbelieve in Him Who created the earth in two days and do you associate partners with Him? He is the Lord of the universes."
Explained in Detail/41:10
He placed on it elements of balance and made it fruitful, and measured and arranged its food in four days for all who seek.
Explained in Detail/41:11
"Then He turned to the smoking sky and said to it and to the earth, "Come (out of chaos) willingly or unwillingly". They said, "We have come willingly."
Explained in Detail/41:12
So He completed them in two days as seven heavens and revealed to each heaven its special laws. And We equipped the lowest heaven with lamps and a system of protection. This is the plan of the Supreme and All-Knowing.