taxable year
The term “taxable year” shall have the same meaning as when used in subtitle A of the Internal Revenue Code of 1986; and the taxable year of any individual shall be a calendar year unless he has a different taxable year for the purposes of subtitle A of such Code, in which case his taxable year for the purposes of this subchapter shall be the same as his taxable year under such subtitle A.
Source
42 USC § 411(e)
Scoping language
for the purposes of this subchapter