IMO The Compliance Decal date isn't going to help narrow anything down a whole lot. All the "Compliance Decal " states is that the vehicle in question was built in compliance with the FEDERAL MOTOR VEHICLE SAFETY STANDARDS that were in Effect on the date shown above. Which means the date on the decal is nothing more than a verification of what safety standards the vehicle and vehicle parts were assembled under. If you have access to all the information to some of the cars built in the first few days of each month and the last few days of the previous month you will find overlapping dates. Imo the date was changed every month due to the possibility of regulation changes since this "Compliance Decal" requirement was in it's infancy. check your Federal Law for changes or simply check "decal" for info changes.