Genesis 1:1-5
1 In the beginning GOD created the heaven and the earth.
2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of GOD moved upon the face of the waters.
3 And GOD said, Let there be light: and there was light.
4 And GOD saw the light, that it was good: and GOD divided the light from the darkness.
5 And GOD called the light Day, and the darkness he called Night. And the evening and the morning were the first Day.
What Calendar did GOD use or create after the rebellion in Eden? Before the fall or rebellion of the population as a whole in Eden, ruled by a Cherub, the time was not counted, for in the Eternity (billions of years:age of the Earth or Universe?) that existed before Genesis 1:1, and is behind us today, there the time was not counted.
GOD started His works of restitution and established a Plan of Restitution or Restoration-Acts 3: 19-26- , so based in the Word of God, the source of the Truth, on GOD's six Days of creation and one Day of rest (a total of seven days) plus the Scriptures that teach that one Day is with the Lord as a thousand years (Psalm 90:4; 2Pet.3:8; Heb. 4:4) so MANKIND, yeah, MANKIND, would go through six GOD Days of 1,000 years each (a total of 6,000 years) plus a Millennium of 1,000 years rest (now a total of 7,000 years).
Actually, Genesis is a Plan of GOD to restitution or restoration of all things-Acts 3:v.17-26-, turning the men to the same nature according they were in the garden of Eden, the Paradise, as is written in Luke 20:35-26 combined with Philippians 3:20-21 among many other biblical references. Take a look.
That said, in according to the biblical Chronology, we can number our days beginning in Genesis chapter 5, as was revealed by GOD to Moses around 2500 years after the rebellion in Eden.
Calendar - Calendly | Pure Bible Forum (and see www.sinaiticus.net )