Not later than 18 months after November 15, 1990, the Administrator, in consultation with the Administrator of the General Services Administration and the Secretary of Defense, shall promulgate regulations requiring each department, agency, and instrumentality of the United States to conform its procurement regulations to the policies and requirements of this subchapter and to maximize the substitution of safe alternatives identified under section 7671k of this title for class I and class II substances. Not later than 30 months after November 15, 1990, each department, agency, and instrumentality of the United States shall so conform its procurement regulations and certify to the President that its regulations have been modified in accordance with this section.