The internet is a little up in arms about determining how old a vehicle actually is. Obviously this plays into 25 year old vehicles. Since most people don't have the first clue even where to look. I'll throw the links and applicable parts.
40CFR85 Subpart X
§85.2302 Definition of model year.Model year means the manufacturer's annual production period (as determined under §85.2304) which includes January 1 of such calendar year, provided, that if the manufacturer has no annual production period, the term “model year” shall mean the calendar year.
(a) Exemption for Non-U.S. Version Vehicles 21 Years Old or Older
The vehicle is 21 OP years old or older and in original unmodified configuration.
● Importer must file with Customs, upon entry, an EPA Form 3520-1 declaring code "E."
● Customs may require proof of vehicle age.
● "OP Years" are determined by subtracting the year a vehicle was manufactured from the
year it was imported.
● The vehicle must be in its original unmodified configuration.
● Vehicles at least 21 years old with replacement engines are not eligible for this exemption
unless they contain equivalent or newer EPA certified engines and emission control
§ 591.4Definitions.All terms used in this part that are defined in 49 U.S.C. 30102, 32101, 32301, 32502, and 33101 are used as defined in those sections except that the term “model year” is used as defined in part 593 of this chapter.
§ 593.4 Definitions.All terms in this part that are defined in section 102 of the National Traffic and Motor Vehicle Safety Act (15 U.S.C. 1391) are used as defined therein.Administrator means the Administrator of the National Highway Traffic Safety Administration.Model year means the year used by a manufacturer to designate a discrete vehicle model irrespective of the calendar year in which the vehicle was actually produced, or the model year as designated by the vehicle's country of origin, or, if neither the manufacturer nor the country of origin has made such a designation, the calendar year (i.e., January 1 through December 31) in which manufacturing operations are completed on the vehicle at its place of main assembly.NHTSA means the National Highway Traffic Safety Administration.
(16) “model year”, when referring to a specific calendar year, means—(A) the annual production period of a manufacturer, as decided by the Administrator, that includes January 1 of that calendar year; or(B) that calendar year if the manufacturer does not have an annual production period.
No person shall import a motor vehicle or item of motor vehicle equipment into the United States unless, at the time it is offered for importation, its importer files a declaration, in duplicate, which declares one of the following:
(i)(1) The vehicle is 25 or more years old.
After more clarification with the US Customs service, they say that "Date of Manufacture" is by Month and Year of Manufacture. Normally this would be a good thing, as cars in the US show a later model year than sometimes when they were produced, but Japan is not like this.