Any person who has been denied bail or who has been unable to obtain bail and who is subsequently imprisoned for an offense committed prior to July 1, 1981, is entitled to commutation of his sentence by the number of days which he spent in a community correctional center from the time he was denied or was unable to obtain bail to the time he was so imprisoned. The Commissioner of Correction shall, if such person has conformed to the rules of the institution, credit such person with the number of days to which the supervising officer of the correctional center where such person was confined while awaiting trial certifies such person was confined between the denial of bail to him or his inability to obtain bail and his imprisonment.