Any area that is initially designated ``attainment/unclassifiable'' for the PM2.5 NAAQS may be subsequently redesignated to nonattainment if ambient air quality data in future years indicate that such a redesignation is appropriate. For any such area that is redesignated to nonattainment for the PM2.5 NAAQS, any absolute, fixed date that is applicable in connection with the requirements of this part is extended by a period of time equal to the length of time between the effective date of the initial designation for the PM2.5 NAAQS and the effective date of redesignation, except as otherwise provided in this subpart.