Most geologists accept radiometric dating techniques as valid because
The passage of time can be charted by the reduction in the number of parent atoms, and the increase in the number of daughter atoms.
Radiometric dating can be compared to an hourglass.
Once all of the sand has fallen out of the top, the hourglass will no longer keep time unless it is turned over again.
When the glass is turned over, sand runs from the top to the bottom.