All methods of radioactive dating relies on some kind of prior assumptions which may not necessarily be true. They are:
1. The rate of radioactive decay and half-lives has remained constant over time. This assumption has the backing of numerous scientific studies and is relatively sound, however, conditions may have been different in the past and could have influenced the rate of decay or formation of radioactive elements.
2. The assumption that the clock was set to zero when the study material was formed. This requires that only the parent isotope be initially present or that the amount of daughter isotope present at the beginning is known so that it can be subtracted.
3. The assumption that we are dealing with a closed system. No loss of either parent or daughter elements has occurred since the study material formed.
Example to show that the zero reset assumption is not always valid: volcanic ejecta of Mount Rangitoto (Auckland, New Zealand) was found to have a potassium-40 age of 485,000 years, yet trees buried within the volcanic material were dated with the carbon-14 method to be less than 300 years old. (. McDougall, A.A. Polach and J.J. Stipp, "Excess Radiogenic Argon in Young Subaerial Basalts From Auckland Volcanic Field, New Zealand," Geochemica et Cosmochemica Acta 33:1485-1520, 1969.)
You can put your trust in a "assumption" but I will not.