((a)) ** Taxable years prior to 1978** For the purposes of determining average monthly wage and quarters of coverage the amount of self-employment income derived during any taxable year which begins before 1978 shall—

((1)) in the case of a taxable year which is a calendar year, be credited equally to each quarter of such calendar year; and

((2)) in the case of any other taxable year, be credited equally to the calendar quarter in which such taxable year ends and to each of the next three or fewer preceding quarters any part of which is in such taxable year.

((b)) ** Taxable years after 1977** Except as provided in subsection (c), for the purposes of determining average indexed monthly earnings, average monthly wage, and quarters of coverage the amount of self-employment income derived during any taxable year which begins after 1977 shall—

((1)) in the case of a taxable year which is a calendar year or which begins with or during a calendar year and ends with or during such year, be credited to such calendar year; and

((2)) in the case of any other taxable year, be allocated proportionately to the two calendar years, portions of which are included within such taxable year, on the basis of the number of months in each such calendar year which are included completely within the taxable year.

((c)) ** Proportional allocation** For the purpose of determining average indexed monthly earnings, average monthly wage, and quarters of coverage in the case of any individual who elects the option described in clause (ii) or (iv) in the matter following for any taxable year that does not begin with or during a particular calendar year and end with or during such year, the self-employment income of such individual deemed to be derived during such taxable year shall be allocated to the two calendar years, portions of which are included within such taxable year, in the same proportion to the total of such deemed self-employment income as the sum of the amounts applicable under for the calendar quarters ending with or within each such calendar year bears to the lower limit for such taxable year specified in .section 411(a)(16) of this titlesection 413(d) of this titlesection 411(k)(1) of this title