Regulations may provide for -
(a) making exceptions from the liability to pay contributions for any specified periods, and
(b) crediting contributions to insured persons for any specified periods, including, in particular -
(i) periods for which there is an exception from the liability to pay contributions by virtue of paragraph (a), and
(ii) the period between the beginning of the contribution year last preceding that in which they become insured persons and their entry into insurance.