It's an interesting question, but it is impossible to place what time Minecraft is taken place, because Minecraft is its own planet with its own timeline (which can't be based off from our own). First off minecraft has to exist in a different universe with different sets of laws, so perhaps its possible for planets to be cubical, and life forms don't require similar elements to survive like we do in our own Universe. Since a planet like Minecraft would need to exist in a different universe with completely different sets of laws, forming any ideas based from what we've learned about the world around us would make it pointless to compare any ideas about what Minecraft really is since our scientific laws don't exist in a world like Minecraft.
The only person that can answer any of your questions is the person who was there to witness the beginning of Minecraft. (AKA Notch)

The rotation speed is also a lot faster then the rotation speed of earth. Idk just something I thought I would note.