A sidebar about helmet buying/"expiration date":
In the grand scheme of things, a helmet that'll be valid for 10 years won't suffer for being a year old when you really need to use it. There'd be 90% life remaining. Not a big deal, and prudence says you might consider replacing it before the final day, anyway.
That, however, depends on your buying a helmet that's as new as possible. If you're shopping in a store you can check for yourself and make sure you get something fresh, but if you're buying online please consider specifying that the helmet must brand new (i.e. valid for the longest possible time) and that you'll return it if it isn't.
Keep that in mind when buying and you likely won't regret getting the helmet early.