I think New Jersy uses stickers on the windshield to show the registration. In California they usually put the year on the sticker (which goes on the rear plate). In some states the plates belong to the car so it gets transferred from owner to owner. This is true in California and depending on the style and number you can estimate the year (but usually they have a fake number for movies and TV). Most states issue the plates in sequential order so you would need to know when a certain pattern started and ended. In other states the plate belongs to the registered owner so they keep it if they sell the car. And in some states, they require you to update your plate every few years to keep everything legible. The DMV is a state authority rather than a federal authority so unless you know each state's rule you can't really figure out the date from the plates.