the present invention relates to method for synchronizing time , apparatus and system . the method discloses an olt generates a passive optical network downstream frame from to an optical network unit , wherein the pon downstream frame comprises a time synchronization message based on ethernet protocol , wherein the time synchronization message comprises a time value that is referenced to a pon media access control control layer and that indicates the onu to adjust a time of a clock coupled to the onu ; and send the pon downstream frame .