(1) Every person who--
         (a) is a qualified person at the commencement of this Act, or
         (b) becomes a qualified person after such commencement,
         shall, if he has not attained the age of thirty years at such commencement, or, as the case may be, on the date on which he becomes a qualified person, be liable until he attains the age of thirty years, to be called up for national service for a period of not more than four years.
     (2) The period of national service for which a qualified person shall be liable to be called up under this Act shall begin from the date on which he is required by an enlistment notice served under this Act to present himself to the authority specified therein and shall end on the day when his term of national service is completed in accordance with the provisions of this Act.