The earth has been determined by radiometric dating to be
We call the original, unstable isotope (Uranium) the "parent", and the product of decay (Lead) the "daughter".
As one example, the first minerals to crystallize (condense) from the hot cloud of gasses that surrounded the Sun as it first became a star have been dated to 4568 plus or minus 2 million years....!! Other events on earth can be dated equally well given the right minerals.
This is an enormous branch of geochemistry called Geochronology.
It is an accurate way to date specific geologic events.
Because geochronologists want to measure isotopes with different masses, a mass spectrometer works really well for dating things.
I do think that radiometric dating is an accurate way to date the earth, although I am a geochronologist so I have my biases.