A person who has been on disability or military leave granted by the board and who wishes to return to active duty in his certified position shall be credited with seniority for the period of such leave and, if otherwise qualified, shall be reinstated to his certified position at the rank or grade held at the start of the leave, not more than 60 days after his written request for reinstatement is filed with the board. Such request shall be filed not more than 30 days after termination of the disability or military or naval service.