Suppose you purify a sample of uranium 238, removing all of the lead. Half of those U-238 atoms will have decayed into lead 4.468 billion years from now. If the lead is removed from that half-uranium/half-lead sample 4.468 billion years from now, half of the U-238 atoms in that purified sample will have decayed into lead in yet another 4.468 billion years. The clock is reset when the decay products are removed. It doesn't matter whether those U-238 atoms were created in one of the very first supernova over ten billion years ago, a supernova shortly before the Earth formed 4.6 billion years ago, or even a supernova that occurred after the Earth formed. Those U-238 atoms do not have a memory.
Some natural processes do this purification and reset the clock when a rock first forms as a solid object. I'll address two; there are others. Zircons readily bond with uranium and thorium but strongly reject lead. Any lead present in zircons now is a result of the radioactive decay of uranium and/or thorium encapsulated within the zircon when it first formed. Potassium-40 sometimes decays into argon-40. The argon simply bubbles out If that decay happens in molten rock. They argon remains trapped if the decay happens inside a solidified crystal. Any argon present in a rock crystal now is a result of the radioactive decay. In both cases, (zircons containing uranium-238 and rock crystals containing potassium-40), the clock was reset when the rock first formed.
