Nuclear decay is a reliable clock because essentially nothing in nature affects it.

The forces inside the nucleus are balanced on a razor's edge. The constants that determine nuclear decay rate are stitched tightly into the fabric of the cosmos. The decay rate can't be changed without changing essentially all of chemistry and physics.

Whole-rock isochron methods measure the time to the last melting of the rock and allow evaluations of initial concentrations. Melting resets the clock since all isotopes are chemically identical. Non-radiogenic isotopes (like strontium-86) provide a reference for the process.

The same whole-rock tests on moon rocks give similar ages.

Lead-lead isochron methods confirm the range of ages. All natural radioactive series of elements end in lead, but there is non-radiogenic lead-204 to serve as a reference. Again, melting resets the clock with reference to lead-204.

An abundance of Antarctic meteorites add confidence to our dating of the solar system. With some 35,000 samples to survey, most date near 4.5 billion years by isochron methods. The few exceptions are of particular interest because they are thought to be fragments blasted off the moon or Mars by impacts.