In order to determine the absolute age (otherwise called the actual age) of a rock layer, Geologists make use of a technique known as absolute dating. This technique is performed by assessing the nuclear decay of radioactive isotopes, a phenomenon that behaves in a clock-like manner.
I would say D. Lamarck thought that organisms changed out of need and after a change in the environment and Darwin thought organisms changed by chance when they were born and before there was a change in the environment.
