Whenever any person is sentenced by an Assam Rifles 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 in which the original proceedings were signed by the presiding officer, or in the case of a Summary Assam Rifles Court, by the Court:
     Provided that when a person subject to this Act is sentenced by an Assam Rifles Court to a term of imprisonment, not being an imprisonment in default of payment of fine, the period spent by him in civil or Force custody during investigation, inquiry or trial of the same case, and before the date of order, such sentence shall be set off against the term of imprisonment imposed upon him and the liability of such person or officer to undergo imprisonment on such order of sentence shall be restricted to the remainder, if any, of the term of imprisonment imposed upon him.