I think this also depends, here in Europe, on legislations. Most legislations will put the first registration year as the year of your car.
For my Pagoda: I have the first registration date (a field in the documents that is required) 01.01.1970. My car was produced March 1970. The reason for the date 01.01.1970 was because the first registration date was not known. So the rule is that they write in Jan 1 of the build year. You may have a variant of similar case if someone put in 01.01.1971 - some detail might have been missing, so they used some rule there.
I have a W463 that was built in 1991 as per the data card, but first registration (in Austria) was in 1993. For various purposes it is either 1991 or 1993 car. The data card is stating order number, but I cannot figure out if the car was made for stock or for customer order. In the registration document I have 1993, as Polish authorities followed the Austrian documents that stated first registration in 1993.
I would say for the classic car that the year of the car is the year when it was built. If you have a new car and you are selling it, then, as the car started being used when it was registered, I would say the date of the first registration (and this is what you have in most of the announcements - the first registration date). Also for guarantee - it starts from the registartion/sale to customer (also dealer), not manufacturing date.
For parts, as said already - it is the VIN and the build year.