«SI Unit of Time (Second)»Resolution 1 of the 13th CGPM (1967/68)(Bureau International des Poids et Mesures (BIPM))jatorrizkotikartxibatua (artxibatze data: 2011-01-10)(kontsulta data: 2015-10-17).
Certain authors caution against identifying "day" with rotation period. For example: Courtney Seligman.Rotation Period and Day Length. (kontsulta data: 2011-06-03).
The average over the last 50 years is about 86,400.002. The yearly average over that period has ranged between about 86,400 and 86,400.003, while the length of individual days has varied between about 86,399.999 and 86,400.004 seconds. See this graph: thumb (data from Earth Orientation Parameters. International Earth Rotation and Reference Systems Servicejatorrizkotikartxibatua (artxibatze data: 26 de abril de 2015).).