The age of rocks can be accurately calculated by measuring the radioactive minerals in them. Radiodating using isochron measurement, proves that the earth is billions of years old.
The Facts Are .....
Radiodating of rocks is based on three assumptions:- (1) the initial conditions are known, (2) no radioactive material has been added since the beginning, and (3) the radioactive decay rate has always been the same. These points can never be proved true as no one was present at the beginning, and no one has assessed the radioactivity at intervals since the beginning. Rock ages calculated from these methods must always be cultivated guess-work. [based on logic]
Isochron age measurements of rocks use such tests as the Strontium/Strontium, Rubidium/
Strontium, Summarium/Niobium & Uranium/Lead relationships. Due to mixing, these tests are meaningless and cannot be used to accurately calculate the age of rocks. There is no way to prove that the amount of radioactive material in the rocks came from the original lava material. 
The US Geological Survey has documented that as much as 90% of the radioactive elements in some granites could be removed by leaching the rock with a weak acid. They also state that as much as 40% of the uranium in fresh-appearing igneous rocks is readily leachable. 
The Committee on the Measurement of Geological Time expressed their lack of confidence in radioactive dating as far back as 1950. They said that the 'dates' were like railway timetables in that they are subject to change without notice. 
"There has been in recent years the horrible realization that radiodecay rates are not as constant as previously thought, nor are they immune to environmental influences. And this could mean that atomic clocks are reset during some global disaster, and events which brought the Mesozoic [era] to a close may not be 65 million years ago but, rather, within the age and memory of man." 
"It is obvious that radiometric techniques may not be the absolute dating methods that they are claimed to be ..... The uncertainties inherent in radiometric dating are disturbing to geologists and evolutionists ....." 
"One serious consequence of the mantle isochron model is that crystallization ages determined on basic igneous rocks by the Rb-Sr whole-rock technique can be greater than the true age by many hundreds of millions of years. This problem of inherited age is more serious for younger rocks, and there are well-documented instances of conflicts between stratigraphic age and Rb-Sr age in the literature."
"Much still remains to be learned of the interpretation of isotopic ages and the realization that in many instances the isotopic age is not necessarily the geological age of a rock has unfortunately led to an over-sceptical attitude by some field geologists." 
- Chemical Geology, Vol. 80, 1989, p:1-16
- K.R. Klepper & D.G. Wyant, "Notes on the Geology of Uranium", US Geological Survey Bulletin, No. 1046-F, 1957 p:93
- "The Penguin Dictionary of Geology", Penguin Books: Middlesex (England), 1972 p:378
- Written in Frederic B. Jueneman, "Secular Catastrophism", Industrial Research and Development, June 1982 p:21
- Written by Dr William D. Stansfield (Instructor of Biology, California Polytechnic State University) in his book "The Science of Evolution", Macmillan: New York, 1977 p:84
- Written by Dr C. Brooks (Professor of Geology, University of Montreal, Canada) and others, in their article "Ancient Lithosphere: Its Role in Young Continental Volcanism", in Science, Vol. 193, September 17, 1976 p:1093
- Written by Peter E. Brown and John A. Miller in their article "Interpretation of Isotopic Ages in Orogenic Belts" in "Time and Place in Orogeny", Geological Society of London Special Publication, No. 3, 1969 p:137