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Radiometric dating methods work something like an hourglass:

Sand in the top = radioactive "parent" isotope
Rate of sand flow = rate of radioactive decay
Sand in the bottom = decay "daughter" isotope The main assumptions behind radiometric dating are: 1. zero reset -The bottom is empty when the "hourglass" is turned over.
2. constant decay rate - The sand keeps flowing at a constant rate.
3. no loss of "parent" - No sand is lost from the top (except flow).
4. no gain of "parent" - No sand is added to the top part.
5. no loss of "daughter" - No sand is lost out of the bottom part.
6. no gain of "daughter" - No sand is added to the bottom part If all these conditions are met, the "hourglass" could tell accurate time. But radiometric dating methods do not usually work this ideally.

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Ó 2010 Arthur V. Chadwick, Ph.D.