https://physics.nist.gov/cuu/Units/second.html http://www.ontologyrepository.com/CommonCoreOntologies/Mid/TimeOntology The Second is used as the basic SI unit of time. Second A Temporal Interval that is equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.