(1) The board shall fix the duration of confinement for persons committed to the custody of the department of corrections under a mandatory life sentence for a crime or crimes committed before July 1, 1984. However, no duration of confinement shall be fixed for those persons committed under a life sentence without the possibility of parole. The duration of confinement for persons covered by this section shall be fixed no later than July 1, 1992, or within six months after the admission or readmission of the convicted person to the custody of the department of corrections, whichever is later.