(a) Life imprisonment for the crime of murder in the first degree pursuant to section ten hundred forty-five or ten hundred forty-five-a of such law;

(b) Life imprisonment for the crime of kidnapping pursuant to section twelve hundred fifty of such law; or

(c) Death commuted to life imprisonment for the crime of murder in the first degree or for the crime of kidnapping pursuant to one of the above sections. Any such person who is not otherwise or who will not sooner become eligible for release on parole under such sentence shall be or become eligible for release on parole after service of a minimum period of imprisonment of twenty years. 2. The provisions of this subdivision shall apply in any case where a person is under one or more of the following sentences imposed pursuant to the penal law in effect prior to September first, nineteen hundred sixty-seven:

(a) A minimum term of twenty years or more and a maximum of natural life for the crime of murder in the second degree pursuant to section ten hundred forty-eight of such law;

(b) A minimum term of twenty years or more and a maximum of natural life for the crime of kidnapping imposed pursuant to section twelve hundred fifty of such law;

(c) A minimum term of fifteen years or more and a maximum of natural life for a third conviction of a felony under laws relating to narcotic drugs pursuant to section nineteen hundred forty-one of such law; or

(d) A minimum term of fifteen years or more and a maximum of natural life for a fourth conviction of a felony pursuant to section nineteen hundred forty-two of such law. Any person who is not otherwise or who will not sooner become eligible for release on parole under such sentence shall be or become eligible for release on parole after service of a minimum period of imprisonment of fifteen years. 3. The provisions of this subdivision shall apply in any case where a person is under a sentence imposed pursuant to the penal law in effect prior to September first, nineteen hundred sixty-seven, other than a sentence specified in subdivisions one and two of this section. Any person who is not otherwise or who will not sooner become eligible for release on parole shall be or become eligible for release on parole under such sentence after service of a minimum period of imprisonment of eight years and four months. Notwithstanding the provisions of subdivisions one and two hereof, inmates convicted of murder, second degree, and sentenced pursuant to the provisions of the penal law in effect prior to September first, nineteen hundred sixty-seven, who are not otherwise or who will not sooner become eligible for release on parole, shall be eligible for release on parole under such sentence after service of a minimum period of imprisonment of eight years and four months. 4. In calculating time required to be served prior to eligibility for parole under the minimum periods of imprisonment established by this section the following rules shall apply:

(a) Service of such time shall be deemed to have commenced on the day the inmate was received in an institution under the jurisdiction of the department pursuant to the sentence;

(b) Where an inmate is under more than one sentence, (i) if the sentences run concurrently, the time served under imprisonment on any of the sentences shall be credited against the minimum periods of all the concurrent sentences, and (ii) if the sentences run consecutively, the minimum periods of imprisonment shall merge in and be satisfied by service of the period that has the longest unexpired time to run;

(c) No credit shall be allowed for "good conduct and efficient and willing performance of duties," under former section two hundred thirty of the correction law, repealed by chapter four hundred seventy-six of the laws of nineteen hundred seventy and continued in effect as to certain inmates, or under any other provision of law;

(d) Calculations with respect to "jail time" "time served under vacated sentence" and interruption for "escape" shall be in accordance with the provisions of subdivisions three, five and six of section 70.30 of the penal law as enacted by chapter ten hundred thirty of the laws of nineteen hundred sixty-five, as amended. 5. The provisions of this section shall not be construed as diminishing the discretionary authority of the board of parole to determine whether or not an inmate is to be paroled.