There is a time when it loses its extra neutrons and becomes C-12.
The loss of those neutrons is called radioactive decay. For carbon, the decay happens in a few thousand years (5,730 years).
Bromine (Br), at atomic number 35, has a greater variety of isotopes. There are two main isotopes at 79 and 81, which average out to the 79.90amu value. While it won't change the average atomic mass, scientists have made bromine isotopes with masses from 68 to 97. As you move to higher atomic numbers in the periodic table, you will probably find even more isotopes for each element.
As you learn more about chemistry, you will probably hear about carbon-14. C-14 is considered an isotope of the element carbon.
By measuring the amount of uranium and ‘radiogenic lead’ in these crystals, one can calculate that, if the decay rate has been constant, about 1.5 billion years must have passed.
(This is consistent with the geologic ‘age’ assigned to the granites in which these zircons are found.) There is a significant amount of helium from that ‘1.5 billion years of decay’ still inside the zircons.
This dating method is based upon the decay of radioactive potassium-40 to radioactive argon-40 in minerals and rocks; potassium-40 also decays to calcium-40.
Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.
Radiometric dating is a method which scientists use to determine the age of various specimens, mainly inorganic matter (rocks, etc.), though there is one radiometric dating technique, radiocarbon dating, which is used to date organic specimens. Basically, scientists take advantage of a natural process by which unstable radioactive “parent” isotopes decay into stable “daughter” isotopes spontaneously over time.