Thebookisaboutdynamicsthemathematicsofhowthingschangeintime.Theuniversearounduspresentsakaleidoscopeofquantitiesthatvarywithtime，rangingfromtheextragalacticpulsationofquasarstothefluctuationsinsunspotactivityonoursun;fromthechangeingoutdoortemperatureassociatedwiththefourseasontotheailytemperaturegluctuationsinourbodies;fromtheincidenceofinfectiousdiseasessuchasmealestothetumultuoustrendofstockprices.