Whenever any person is sentenced by a Security Guard 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 Security Guard Court, by the Court:
     Provided 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 on 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.