Determination of time intervals on a geologic scale, through either absolute or relative dating methods. Absolute dating methods involve the use of radioactive elements and knowledge of their rates of decay: this yields an actual age in years for a given rock or fossil. Relative dating involves the use of fossils or sediments to place events and rock sequences in order, and does not provide absolute dates. See also dating methods; absolute age; relative age; planktonic geochronology; geochronometry.