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).
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.However, the error range increases drastically once you pass 50,000 years.Also, it is of little use in anything more recent than 5,000 years ago.Of course C-14 would never be of any use for dating dinosaur bearing deposits, unless you want everything to date to around 40,000 years!
Radio-Carbon dating can be used for dates up to ~80,000 years ago.
Fortunately, we are able to date older fossils using the radiometric breakdown of other elements (Potassium-Argon dating, Argon-Argon dating, and Rubidium dating [I'm writing this without any refs - so this last one might be wrong]).
Usually the radioactive 'clocks' for these elements are started when the elements are deposited by a volcanic eruption (usually in the form of ash).
About half of the half of the original amount (1/2 * 1/2 = 1/4) of U-235 has decayed into other materials - meaning that only half of its half life has passed - therefore: ~300 mya.
Other forms of dating are: The most common geological methods of dating are the decay of Uranium into Lead, a natural process that occurs in Uranium ore, and the Potassium-Argon method, useful with volcanic deposits.
Carbon 14's half-life is not nearly long enough to measure dates in the geological past.