Mont. Code § 75-2-428
In the case of any emission limitation, emission standard, or other requirement approved or adopted by the department under this chapter after July 1, 1979, and approved by the federal environmental protection agency as an amendment to the state implementation plan, which is more stringent than the emission limitation or requirement for the source in effect prior to such approval or promulgation, if any, or where there was no emission limitation, emission standard, or other requirement approved or adopted before July 1, 1979, the date for imposition of the noncompliance penalty under 75-2-421 through 75-2-429 shall be the date on which the source is required to be in full compliance with such emission limitation, emission standard, or other requirement or 3 years after the approval or promulgation of such emission limitation or requirement, whichever is sooner.
§ 75-2-428, MCA