Even in the case of very long half-lives, modern scientific instruments are now accurate enough to give very fine readings.
We usually hear of Carbon 14 dating, which is very important in archaeology.
(The item being tested must be organic based, and must be dead - tests on live mollusks showed an age of 2000 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.
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.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).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.During the 19th century, and even well into the twentieth, geological chronology was very crude.