Thank you for the question on calendar, “Where was the calendar first developed according to Genesis 8:12-13?”
According to Genesis 1, God made it possible for man to understand the seasons and times. Thus, the “calendar” was not developed in Genesis 8 but in Genesis 1.
Genesis 1 says this, “14 And God said, “Let there be lights in the vault of the sky to separate the day from the night and let them serve as signs to mark sacred times, and days and years, 15 and let them be lights in the vault of the sky to give light on the earth.” And it was so” (NIV).
Even though I don’t believe that the early humans were cavemen, certainly they would observe the sunrise and sunset, the shortening of the seasons, the changing weather and temperature and adjusted their living as such. They were smart enough to live in caves during the harsh seasons and from the wild animals. They would observe the migration of birds over time that would indicate a change of season is coming. They may not have the months but certain they knew when Spring, Summer, Fall, and Winter by watching the plants and animals.
Genesis 8 affirms God’s statement in Genesis 1. With the climate change from the Flood, the seasons were more accentuated on earth. Depending on where on live on the landmass, the descendants of Adam and Eve would be able to observe the heavens and life on earth. It would be reasonable to think that the chief or priest of a people group would be the watchers of the heavens and of the seasons.
Archaeologists has now found a “temple” sight that is even older than the Sumerians (4500 BCE, Stonehenge, 1800 BCE, even the Egyptians (3500 BCE). It’s Gobekli Tepe, dating to 9500 BCE with its monolithic structures. That’s when archaeologists dated that site, but there was something going on before that structure was made.
The first calendar was observed by Adam and Eve as they lived out their years and their descendants, seeing the repeated patterns of nature. They may not call it a “calendar,” but they knew the seasons.
SUMMARY: Genesis 8 is not the first calendar, but when God made Adam and Eve. They observed the pattern of nature and its seasons.
-Kingston