Ex Nihilo

When God made the earth in seven days, He didn't make certain things without also giving them the ability to be made in additional quantities, in future, by nature.

For example, when He made fruit trees, He gave them the ability to be reproduced in multiple quantities in future, by giving them seeds.

When He made animals, He gave them the ability to be made in multiple quantities in future, by biological reproduction.

And it's the same with geology. When He made stones, oceans, rivers and mountains He also set in place the ability for more of the same to be made in future, through various geological processes.

Same with stars. He made stars in one day - and He likely also made it possible for new stars to form.

Remember, God may have originally intended for the earth to remain for quite a long time, so it's no wonder He didn't only make things but also gave the ability for some of those things to be made in additional or multiplied numbers through nature as time went on.

He originally made everything in seven days - but the making of more of those things now through differing processes might require longer and differing amounts of time.

We can study the processes and learn the required amounts of time for each thing to be made, but that can't tell us when the original ones were made because the original ones weren't made through that process and didn't require that length of time - they were made through the Word of God, ex nihilo (out of nothing), in a day.