I'm not for sure if thats even possible. The last 5 digits were only for the # of that specific car per plant. Mabye someone else knows how exactly or even if it's possible.
http://pontiacsafari.com/
Go here under Pontiac facts and information and than go under the identification tags section. It has a very good description on how to decipher cowl tags and serial tags. But it doesn't mention about build date info.