(a) An indeterminate sentence of imprisonment commences when the prisoner is received in the custody or institution to which he was sentenced.
(b) A definite sentence of imprisonment commences when the prisoner is received in the custody to which he was sentenced. Where a person is under more than one definite sentence, the sentences shall be calculated as follows: (1) If the sentences run concurrently, the terms merge in and are satisfied by discharge of the term which has the longest term to run; (2) if the sentences run consecutively, the terms are added to arrive at an aggregate term and are satisfied by discharge of such aggregate term.
(c) When a sentence of imprisonment that has been imposed on a person is vacated and a new sentence is imposed on such person for the same offense or for an offense based on the same act, the new sentence shall be calculated as if it had commenced at the time the vacated sentence commenced, and all time served under or credited against the vacated sentence shall be credited against the new sentence.
(d) When a person who is serving a sentence of imprisonment escapes, the escape shall interrupt the sentence and such interruption shall continue until the return of such person to the custody of the Commissioner of Correction.