Sec. 15. COMPUTATION OF PERIOD OF SERVICE. In computing the twenty-five (25) years of service required for a retirement pension, twenty-five (25) years of continuous service shall be required; provided, however, that in case of interruption of less than two (2) years, credit shall be given for previous service if out of service more than two (2) years; no service prior to said interruption shall be counted, other than provided in Section 21.