There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate.As one example, the first minerals to crystallize (condense) from the hot cloud of gasses that surrounded the Sun as it first became a star have been dated to 4568 plus or minus 2 million years....!! Other events on earth can be dated equally well given the right minerals.
Kieth and Anderson show considerable evidence that the mussels acquired much of their carbon from the limestone of the waters they lived in and from some very old humus as well.
This is an enormous branch of geochemistry called Geochronology.
It is an accurate way to date specific geologic events.
This assumption is backed by numerous scientific studies and is relatively sound.
However, conditions may have been different in the past and could have influenced the rate of decay or formation of radioactive elements.
The older an organism's remains are, the less beta radiation it emits because its C-14 is steadily dwindling at a predictable rate.