Whenever any person is sentenced by a Force Court under this Act to imprisonment, the term of his sentence shall, whether it has been revised or not, be reckoned to commence on the day on which the original proceedings were signed by the presiding officer, or in the case of a Summary Force Court, by the Court:
     Provided that if for any reason beyond the control of the commanding officer or superior officer, the sentence of imprisonment cannot be executed in full or in part, the convict shall be liable to undergo the whole or unexpired portion of sentence, as the case may be, when it becomes possible to carry out the same:
     Provided further that the period of detention or confinement, if any, undergone by an accused person during the investigation, inquiry or trial of the case in which he is sentenced and before the date of which the original proceedings were signed shall be set off against the term of his sentence and the liability of such person to undergo imprisonment shall be restricted to the remainder, if any of the term of his sentence.