Each subpart contains a section which specifies the latest dates by which national standards are to be attained in each region in the State. An attainment date which only refers to a month and a year (such as July 1975) shall be construed to mean the last day of the month in question. However, the specification of attainment dates for national standards does not relieve any State from the provisions of subpart N of this chapter which require all sources and categories of sources to comply with applicable requirements of the plan -
(a) As expeditiously as practicable where the requirement is part of a control strategy designed to attain a primary standard, and
(b) Within a reasonable time where the requirement is part of a control strategy designed to attain a secondary standard.