Doesn’t that prove the world is millions of years old?
We don’t know what happened to the rock during its ‘lifetime’.
An hour glass is only useful if it is not disturbed.
Unlike the hourglass, we do not know how much of each isotope was in the rock in the beginning.
That’s because we did not observe what happened in the past when the rock formed.
There is an isotope of samarium that decays into neodymium, and one of rubidium that decays into strontium.
Radioactive dating is often illustrated with an hour glass.
It came from a geologic philosophy, not a scientific measurement.
Note too that radioactive dating is something that most people don’t understand.
But after rocks crystallize from molten magma, they can be heated and cooled; they can be affected by metamorphic events and groundwater.