Your snail-journey dating method relies on your knowing two facts. The snail's starting point. The fact that nothing has impeded or assisted the snail's progress at any point. It is not possible for you to know these two points for certain. They are therefore not facts at all, but assumptions.
Similar assumptions underpin the use of radioactive isotopes for dating methods.
Carbon exists naturally in 3 isotopes. Both 12 C and 13 C are stable atoms. However, this leaves approximately 1 part per million of all carbon atoms, as 14 C, which is radioactive. Radioactivity is a statistical phenomenon. This is rather like the chance of throwing a 6 on a die.