Those who adopt this line of thinking usually believe in what's called the "Gap Theory." They believe, you pointed out, that the earth can really be millions of years old and that the current earth age is only 6,000 years old.
This theory is just that, a theory. Most learned theologians have debunked that line of thinking as pure speculation.
The Bible leaves it open for discussion. There’s strong Biblical reasons to go with the Gap Theory but to each his own.
The real point here is the issue in the debate between creationism and Darwinism is not the age of rocks but the origin of living things.