Our Moon formed about 4.5 billion years ago when the proto-Earth was blasted by another, somewhat smaller but quite large protoplanet. The excavated material that remained in orbit around the Earth then coalesced into the Moon. The time of the formation can be quite reasonably determined through the dating of the oldest rocks that came into existence. As the material of the Moon cooled down, lighter minerals floated to the top and formed a solid crust, creating the rocks that tell the story of the first times. According to calculations, the upper layers solidified in only a thousand years, creating a lid that slowed down the cooling of the lower layers dramatically to ten million years. Though events like massive impacts and volcanism may've altered the age of the surface materials here and there, almost all of the oldest rocks formed during these mere ten million years. Or did they?
Rock sample 60025, collected by the crew of Apollo 16, while still on the Moon.