Dating takes two forms: it can deliver an absolute age in years or an age relative to other events. Relative dating is sometimes all that is available to us.
All geological dating methods come with a small amount of uncertainty, because they rely on probabilities and inference, based on the best available data.
Some dating methods can be interfered with by contamination of the sample, but combining several methods can help avoid the risk of error from this source.
The oldest fossil traces we know of go back to about 3800 million years, but as most rocks of that age have been since destroyed, life may be a little older.
In 1920, Andrew Douglass suggested dendrochronology, using tree rings to build a sequence of years, and using other timber with overlaps to extend the scale.
Dendrochronology can be used to date artefacts very accurately for thousands of years, relying on unique patterns that can be traced from one tree to another.
In 1947, Willard Libby introduced the idea of carbon-14 dating. By 1949, he could present carbon dating as a fully developed technique, ready to use.
Material less than 50,000 years old can be dated by carbon dating, provided it has organic material which has not been contaminated since it was formed.
Thermoluminescence can identify how long some things have been buried. The thermoluminescence clock is 'reset' when the objects are exposed to direct sunlight.
Ice cores provide good evidence of past climates and temperatures. The cores preserve stable isotope ratios in water and gases, and solids like volcanic ash.
Isotope dating works with many igneous rocks, and this can be used to determine absolute limits to the age range of fossils lying between two igneous layers.