Simply counting the number of rings will give one a fairly good idea of the age of the tree.
Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings. When a given quantity of an isotope is created (in a supernovae, for example), after the half-life has expired, 50% of the parent isotope will have decomposed into daughter isotopes.
Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fall and winter months.
When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings.
This element is locked in tiny zircons within the granite. While it stays within the zircon for a period of time, being a very small atom, helium escapes the zircon within a few thousand years.
This means the above calculations are only evolution speculation and NOT backed up by real science. Wiens has a Ph D in Physics, with a minor in Geology.His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.Clouds formed and storms raged, raining more and more water down on the primitive earth, cooling the surface further until it was flooded with water, forming the seas.It is theorized that the true age of the earth is about methods to determine the age of rocks means scientists have to rely on when the rock was initially formed (as in - when its internal minerals first cooled).