This is a method that is used to date rocks that are believed to be very old. Rb-87 decays to Sr-87, with a half-life of 48.8 billion years. Rb is chemically similar to K, and Sr is chemically similar to Ca. All the Rb is not eliminated at mineral formation, so there is no "zero reset". In order to calculate (and correct for) the amount of Sr-87 at the beginning, full mixing of all Sr isotopes at mineral formation has to be assumed.
2010 Arthur V. Chadwick, Ph.D.