Older dates may change by a few million years up and down, but younger dates are stable.
For example, it has been known since the 1960s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old.
The methods are all based on radioactive decay: The first radiometric dates, generated about 1920, showed that the Earth was hundreds of millions, or billions, of years old.
Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.
Every few years, new geologic time scales are published, providing the latest dates for major time lines.
A key point is that it is no longer necessary simply to accept one chemical determination of a rock’s age.
Age estimates can be cross-tested by using different isotope pairs.
Current understanding of the history of life is probably close to the truth because it is based on repeated and careful testing and consideration of data.
The rejection of the validity of fossils and of dating by religious fundamentalists creates a problem for them: Fossil sequences were recognized and established in their broad outlines long before Charles Darwin had even thought of evolution.
If the fossils, or the dating of the fossils, could be shown to be inaccurate, all such information would have to be rejected as unsafe.