In crediting quarters of coverage under §404.143(a), we credit self-employment income you derived during a taxable year that begins after 1977 to calendar years as follows:

(a) If your taxable year is a calendar year or begins and ends within the same calendar year, we credit your self-employment income to that calendar year.

(b) If your taxable year begins in one calendar year and ends in the following calendar year, we allocate proportionately your self-employment income to the two calendar years on the basis of the number of months in each calendar year which are included completely within your taxable year. We consider the calendar month in which your taxable year ends as included completely within your taxable year.