Virtually nothing can influence the rate of such radioactive decay, so it can represent a very accurate timing method for physical processes.
There are many physical phenomena that occur at a rate that depends on how much of the material exists at any time.
The rate of radioactive decay is dependent on the number of remaining undecayed atoms still remaining at any moment.
These are generally referred to as "exponential growth" or "exponential decay".
The most famous of these methods uses Carbon-14, and a complete discussion is presented here in order to fully understand WHY it is pretty accurate and why, when careful science is done, the results are very reliable.