The Geological Timescale divides the Earth’s history into several periods of differing lengths of time.There are different ways that scientists can measure geological time.
Do you honestly think that no one has done anything about it? By making thousands (if not millions) of these adjustments we get a very good idea of how old a piece of unknown material can be. The 2009 calibration set extends the ‘well calibrated range’ to 50,000 years using the varves in a Japanese lake. This is unlike the creationists which think it happened, but can’t be bothered to check.
The first is that atoms have always decayed at the same rate.
And this isn’t really an assumption as the decay rates have been tested in the laboratory for a hundred years or so, we have an example of a natural nuclear reactor where we can measure the various products and determine the decay rates (and the fine structure constant), and we can observe the past by looking deep into the past of the universe. The sigh isn’t for the effort of writing, it’s for the effort of finding all the references.
By measuring the amount of the parent and daughter isotopes in a crystal, and then applying the decay rate, the actual age in years since the rock crystallized can be calculated.
Check out this video on the Uranium – Lead dating method: Biostratigraphy is a relative dating method that correlates rock ages using the fossils contained within rock units.