This more than 20-billion-year timeline of our universe shows the best estimates of major events from the universe's beginning to anticipated future events. Zero on the scale is the present day. A large step on the scale is one billion years; a small step, one hundred million years. The past is denoted by a minus sign: e.g., the oldest rock on Earth was formed about four billion years ago and this is marked at -4e+09 years. The "Big Bang" event most likely happened 13.8 billion years ago; see age of the Universe.