Long answer: The Earth formed together with the rest of the Solar System and its meteorites around 4.5 billion years ago.
U would have been stable in the crystallographic site, but the site is now occupied by by Pb.
The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.
We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.
The dating equation used for K-Ar is: Carbon Dating Radiocarbon dating is different than the other methods of dating because it cannot be used to directly date rocks, but can only be used to date organic material produced by once living organisms.
Some background: We are able to determine the age of certain rocks and minerals using measurements of radioactive and radiogenic isotopes of certain elements. Simply put, the resulting date is the time that has passed from the crystallisation of that mineral.