[Physicist]

Science assumes billions of years. Science assumes evolution. It hypothesizes how this occurred and then looks for the evidence.

Codswallop. In the 19th century, it was believed that the Earth was no more than 30 million years old. The geological evidence that the Earth was several billion years old came as a bolt from the blue. Far from an assumption, the fact of Earth's enormous age obtruded itself in spite of what the theorists believed.

[DittoJed2]

The years are proposed periods of times. They were proposed by men who are fallible. Dating methods do not confirm them and circular reasoning is often used (we date the layers by the fossils and the fossils by the layers). It is hypothesis.

[DittoJed2]

The theory "evolved" from the philosophies of a handful of men, most famously James Hutton, who was picked up by Lyell, who was picked up by Darwin. It is still an evolving theory.