Scientists know that the layers they see in sedimentary rock were built up in a certain order, from bottom to top.
When they find a section of rock that has a lot of different strata, they can assume that the bottom-most layer is the oldest and the top-most layer is the youngest.
One of the biggest jobs of a geologist is establishing the absolute age, in years, of a rock or fossil.
He could be pretty confident that his super awesome dinosaur was about 175 million years old.
Stratigraphic and fossil succession are good tools for studying the relative dates of events in Earth's history, but they do not help with numerical dating.
Again, this doesn't tell them exactly how old the layers are, but it does give them an idea of the ordered sequence of events that occurred over the history of that geologic formation.
Sort of an offshoot of stratigraphic succession is fossil succession, or a method in which scientists compare fossils in different rock strata to determine the relative ages of each.
In 1905, Ernest Rutherford figured out that we could use radiation to establish the ages of rocks.