Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.
Unfortunately, these elements don't exist in dinosaur fossils themselves.
Some evolutionists throw out theistic evolution (God using evolution as His creative process) as a philosophical panacea, with the goal of leading people to conclude that Genesis is a myth.
Like Nimrod of ancient times, they know they must provide an alternative (i.e., naturalism, specifically —the belief that science alone can render truth about our world and reality) to biblical truth if they are to hold sway over the public in what is essentially a couched rebellion against God.
If, in the year AD 1600, you had asked an educated European how old the planet Earth was and to recount its history he would have said that it was about 6000 years old and that its ancient history was given by the biblical account in Genesis.