40 CFR §93.163

1. (a)The emissions reductions from an offset or mitigation measure used to demonstrate conformity must occur during the same calendar year as the emission increases from the action except, as provided in paragraph (b) of this section.
2. (b)The State or Tribe may approve emissions reductions in other years provided:
   1. (1)The reductions are greater than the emission increases by the following ratios:
   2. (2)The time period for completing the emissions reductions must not exceed twice the period of the emissions.
   3. (3)The offset or mitigation measure with emissions reductions in another year will not:
      1. (i)Cause or contribute to a new violation of any air quality standard,
      2. (ii)Increase the frequency or severity of any existing violation of any air quality standard; or
      3. (iii)Delay the timely attainment of any standard or any interim emissions reductions or other milestones in any area.
3. (c)The approval by the State or Tribe of an offset or mitigation measure with emissions reductions in another year does not relieve the State or Tribe of any obligation to meet any SIP or Clean Air Act milestone or deadline. The approval of an alternate schedule for mitigation measures is at the discretion of the State or Tribe, and they are not required to approve an alternate schedule.