Except as provided in subsection (b), not later than 60 days after August 10, 2012, the President shall impose 5 or more of the sanctions described in section 6(a) of the Iran Sanctions Act of 1996, as amended by section 204, with respect to a person if the President determines that the person knowingly, on or after August 10, 2012, provides underwriting services or insurance or reinsurance for the National Iranian Oil Company, the National Iranian Tanker Company, or a successor entity to either such company.