Different world-views have different time-scales as regards the history of the Earth and the universe.Buddhist, Jain and Hindu cosmologies describe the universe as a never-ending series of cycles each lasting millions or billions of years.
Astronomy and Geology, working on scientific data built up over the last two centuries or so, assumes the Earth to be very ancient, more in keeping with the eastern than the western (Graeco-Judeo-Christian) perspectives.
However, whereas the Eastern perspective is (like the Greek) cyclic, the Western scientific understanding is strictly linear and evolutionary.
During the 19th century, and even well into the twentieth, geological chronology was very crude.
Dates were estimated according to the supposed rate of deposition of rocks, and figures of several hundred million years were bandied out; usually arrived at through inspired guesswork rather than anything else.
With the discovery of radiometric dating, it became possible for the first time to attempt precise figures.
Radiometric dating works on the principle that certain atoms and isotopes are unstable.
These unstable atoms tend to "decay" into stable ones; they do this by emmitting a particle or particles. The time it takes for half of a given amount of a radioactive element to decay into a stable one is what is known as the "half-life".
By matching the proportion of original unstable isotope to stable decay product, and knowing the half-life of that element, one can thus deduce the age of the rock , as shown in the follwoing diagram.
Even in the case of very long half-lives, modern scientific instruments are now accurate enough to give very fine readings.