California Code, Penal Code - PEN § 2931

(a) In any case in which a prisoner was sentenced to the state prison pursuant to Section 1170, or if he committed a felony before July 1, 1977, and he would have been sentenced under Section 1170 if the felony had been committed after July 1, 1977, the Department of Corrections shall have the authority to reduce the term prescribed under such section by one-third for good behavior and participation consistent with subdivision (d) of Section 1170.2.  A document shall be signed by a prison official and given to the prisoner, at the time of compliance with Section 2930, outlining the conditions which the prisoner shall meet to receive the credit.  The conditions specified in such document may be modified upon any of the following:

(b) Total possible good behavior and participation credit shall result in a four-month reduction for each eight months served in prison or in a reduction based on this ratio for any lesser period of time.  Three months of this four-month reduction, or a reduction based on this ratio for any lesser period, shall be based upon forbearance from anyact for which the prisoner could be prosecuted in a court of law, either as a misdemeanor or a felony, or any act of misconduct described as a serious disciplinary infraction by the Department of Corrections.

(c) One month of this four-month reduction, or a reduction based on this ratio for a lesser period, shall be based solely upon participation in work, educational, vocational, therapeutic or other prison activities.  Failure to succeed after demonstrating a reasonable effort in the specified activity shall not result in loss of participation credit.  Failure to participate in the specified activities can result in a maximum loss of credit of 30 days for each failure to participate.  However, those confinedfor other than behavior problems shall be given specified activities commensurate with the custodial status.

(d) This section shall not apply to any person whose crime was committed on or after January 1, 1983.