God had already created the universe, including a lifeless planet Earth, by the time the first creative day began.
Evidently the six creative days were long periods during which God prepared the earth for human habitation.
The Bible account of creation does not conflict with scientific conclusions about the age of the universe.
However, your Old Earth Creationism still disagrees with the evidence. For example, not only did dinosaurs live tens of millions of years ago, but they died out tens of millions of years ago. The trees bearing fruit were supposed created on the third day-age, but such trees requires flying insects to pollinate them, and flying things were created on the fifth day.
In reality, there were creatures in the sea before there were plants on land, and creatures on the ground before there were flying creatures.