I do not see why it matters anyways. So would probably just ask why it is supposedly 5 billion years old, and how that is proven. Then they say something about carbon13, and I ask how it can be so pinpointed to decades millions of years ago. They kind of shortcircuit after that, because they just parrot. I never believed in that accuracy as an atheist either. Then ask what does it matter how old the earh is, does not change anything for the human life. At this point, is just a thing to mock christians with, for them to feel good about themselves. Just ask them why it matters, and they have no reply or try to improvise some nonsense that is most often a big lie, easily picked apart.