Answer 2: Yes, radiometric dating is a very accurate way to date the Earth.
We know it is accurate because radiometric dating is based on the radioactive decay of unstable isotopes.
We call the original, unstable isotope (Uranium) the "parent", and the product of decay (Lead) the "daughter".
It is commonly used in earth science to determine the age of rock formations or features or to figure out how fast geologic processes take place (for example, how fast marine terraces on Santa Cruz island are being uplifted).
Radiometric dating relies on the principle of radioactive decay.
There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate.
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.
Obviously, if the substance you are measuring is contaminated, then all you know is the age since contamination, or worse, you don't know anything, because the contamination might be in the opposite direction - suppose, for example, you're looking at radio carbon (carbon 14, which is produced in the atmosphere by cosmic rays, and which decays into nitrogen).
Since you are exposed to the atmosphere and contain carbon, if you get oils from your skin onto an archeological artifact, then attempting to date it using radio carbon will fail because you are measuring the age of the oils on your skin, not the age of the artifact.
We have also obtained a very similar age by measuring Pb isotopes in materials from earth.
I should mention that the decay constants (basically a value that indicates how fast a certain radioactive isotope will decay) for some of these isotope systems were calculated by assuming that the age of the earth is 4.56 billion years, meaning that we will also calculate an age of 4.56 billion years if we use that decay constant.
For example, the element Uranium exists as one of several isotopes, some of which are unstable.
When an unstable Uranium (U) isotope decays, it turns into an isotope of the element Lead (Pb).
This is an enormous branch of geochemistry called Geochronology.