You can prove beyond a reasonable doubt that the Earth is billions of years old. It's been proven.
Maybe God made the earth billions of years ago....but didn't make man until much later.
The Bible doesn't say how long it was between making Earth and making man...there could be YEARS missing there.
Actually it does. Day 3 is Earth. Day 6 is Man.
But how many "possible" years are there between one day and the next? It could have taken, in our time, 6 billions years between the 1st day and the 6th.