20 C.F.R. § 404.142

How we credit self-employment income to calendar quarters for taxable years beginning before 1978.

            In crediting quarters of coverage under § 404.141(b)(2), we credit any self-

            employment income you derived during a taxable year that began before 1978 to calendar quarters as follows:

        (a) If your taxable year was a calendar year, we credit your self-employment income equally to each quarter of that calendar year.

        (b) If your taxable year was not a calendar year (that is, it began on a date other than January 1, or was less than a calendar year), we credit your self-employment income equally—

        (1) To the calendar quarter in which your taxable year ended; and

        (2) To each of the next three or fewer preceding quarters that were wholly or partly in your taxable year.

Official source