Radiometric dating instruments
Tree ring data (dendrochronology) can be used to even out this inconsistency, however the oldest trees used for calibration are in the order to about 6,000 years old, so any further back than that and you can't correct your dates (although there are reportedly some preserved huon pines in Tasmania that could take this right back to 30,000 years or so, if anyone wants to spend half their life time counting tree rings).Even if dates are corrected with tree ring data they are still not considered calendar years, but rather radiocarbon years.The radioactive isotope Carbon 14 has a half-life of 5,730 years.This has made it useful for measuring prehistory and events occurring within the past 35 to 50 thousand years.The Christian Creationists have criticized it on the grounds that it is inaccurate.But these inaccuracies are the result of variation in the level of Carbon 14 in the atmosphere, and when this is worked out (through calibration with tree rings of the bristlecone pine, the oldest living organism) precise dates can be had.
If a fossil is completely replaced (permineralized), then it would be useless in a similar test - because it no longer is organic.
For example, Uranium (U-235 or U-238) runs into the Thorium series then breakdowns into Radium and Radon, and finally, into Lead (the stable isotope).
Volcanic tuft containing U-235 also contains (stable) Lead associated directly with it.
By comparing the proportion of the two, one can work out how old the deposit is.
If the sample is 75% U-235 and 15% Lead (and 10% other), then the sample is approximately 300 mya.