Every person convicted of a felony before October 1, 1942, who had once before been convicted of a felony and regularly discharged from the state corrections system, or who, prior to June 24, 1944, had been returned to a state correctional facility for violating the terms of a conditional pardon, or who had been convicted of a crime while serving his sentence in a state correctional facility, or who had escaped or attempted to escape from a state correctional facility or from a local correctional facility while awaiting trial or transfer to a state correctional facility, shall, for every month he is confined in any state correctional facility after such date, without violating any prison rule or regulation, be allowed a credit of fifteen days upon the total term of confinement to which he has been sentenced, in addition to the time he actually serves. Every person convicted of a felony before October 1, 1942, who is returned thereafter to a state correctional facility for violating the terms of a conditional pardon, or who commits a crime while serving his sentence in a state correctional facility, or who escapes or attempts to escape from a state correctional facility, shall, for every twenty days he is held in confinement after his return to a state correctional facility or after the commission of such crime, or after such escape or attempted escape, without violating any prison rule or regulation, be allowed a credit of only ten days upon the total term of confinement to which he has been sentenced, in addition to the time he actually serves. Any credit allowed under the provisions of this section shall also be considered as reducing the term of imprisonment to which the prisoner was or is sentenced for the purpose of determining his eligibility for parole. So much of an order of any court contrary to the provisions of this section shall be deemed null and void.