Section 129D. Prisoners are eligible to earn deductions from sentences and completion credits, collectively known as good conduct deductions, for participation in and completion of programs and activities as follows:

(a) For the satisfactory conduct of a prisoner while confined at a correctional institution of the commonwealth, or any jail or house of correction, but working at a state hospital or state school, satisfactory completion of an educational program leading to the award of a high school equivalency certificate, satisfactory performance of said prisoner in completion of any other educational sequence or any vocational training program established within or without the institution, satisfactory performance of said prisoner while the prisoner is employed on work-release or in a prison industry, or satisfactory performance of said prisoner in any other program or activity which the superintendent of the institution shall deem valuable to said prisoner's rehabilitation, the commissioner may grant, in addition to the deductions of sentence provided under section 129C, a further deduction of sentence pursuant to this section. For a prisoner serving a sentence to the state prison, such deduction shall not exceed 7.5 days per program or activity for each month while said prisoner is working in a state hospital or school, on work-release or working in a prison industry or partaking in any of the said programs or activities as aforesaid; provided, however, that in no event shall said deductions exceed a maximum monthly total of 15 days. For a prisoner serving a sentence to the house of correction, such deduction shall not exceed 5 days per program or activity for each month while said prisoner is working in a state hospital or school, on work-release or working in a prison industry or partaking in any of the said programs or activities as aforesaid; provided, however, that in no event shall said deductions exceed a maximum monthly total of 10 days. Further, the commissioner may grant an additional deduction of sentence of up to 10 days for a prisoner's successful completion of a program or activity, as designated by the commissioner, to be deducted in the month during which successful completion of the designated program or activity is achieved; provided, however, that for a prisoner serving a sentence to the house of correction, such additional deduction of sentence shall be granted only for completion of a program or activity requiring 6 months of satisfactory participation.

(b) All such deductions of sentence shall be added to any deduction to which the prisoner is entitled under section 129C for reducing the term of imprisonment by deduction from the maximum term for which the prisoner may be held under the prisoner's sentence or sentences; provided, however, that in no event shall such deductions reduce the imposed maximum term or aggregate maximum terms by more than 35 per cent.

(c) In addition to the foregoing, the commissioner may also grant up to 80 days of completion credits to a prisoner serving a sentence to the state prison for successful completion of a program or activity, as designated by the commissioner, to be granted in the month during which successful completion of the designated program or activity is achieved; provided, however, that in no event shall the aggregate number of completion credits awarded to a prisoner exceed a maximum of 17.5 per cent of such prisoner's imposed maximum term of imprisonment.

(d) Such deductions granted under subsection (a) and such completion credits granted under subsection (c) shall be added to any deduction to which the prisoner is entitled under section 129C for reducing from the minimum term of the sentence or sentences the good conduct credits earned under this section for parole eligibility as provided under section 133; provided, however, that in no event shall said deductions and such completion credits reduce such imposed minimum term by more than 35 per cent.

(e) No prisoner shall be eligible for any deduction under subsection (a) or any completion credit under subsection (c) unless the prisoner has satisfied both the requirements of the program or activity and demonstrated competency in the material, as determined by the commissioner.

(f) A prisoner whose term of imprisonment is reduced from the maximum term for which the prisoner may be held under the prisoner's sentence or sentences shall receive from the commissioner a certificate of discharge on the date which has been determined by such additional deductions from the maximum term of the prisoner's sentence or sentences.