The origins of atheistic millions and billions of years can be traced back to Charles Lyell, a lawyer who wrote a book called “Principle of Geology” in 1857. His main point was the assumption that the present is the key to the past. He just arbitrarily decided that only the current natural processes can explain the past.
Since the current natural processes are slow and gradual, he speculated that the past processes had to be slow and gradual. How could he possibly know this? The atheist Charles Darwin who was a friend of Lyell bought into these millions of years and quickly applied these millions of years to humans, animals and plants.
They just made this up without any proof whatsoever. This is just their opinion, and their main motivation was to promote atheism.
Today, a majority of scientists from most scientific fields start with this assumption or guess that the present is the key to the past when dating the universe, Earth and all life. This is all based in the atheistic worldview and not science. It amazes me that people can be stupid enough to believe anyone who thinks everything happened by time and chance and not by an all-knowing, all-powerful eternal creator.