Hi Blueberry,
you will find the build date:
1. Open the driver side door, there should be a sticker with the build date.
or
2. The Fender Tag, there should be a code with 3 digits, see photo.
In this case 820 = This is August 20th
or
3. The broadcast sheet, if you have that.
If you can't find the build date, please don't worry, you can still register your car, just leave the build date box blank.
As soon there is another car in the registry with a VIN Number very close to yours, you will know the build date of your car, or at least plus or less a few days.
Best regards, JJ