New date time api is already in java 8 and it is really cool. So you can use it right now. http://docs.oracle.com/javase/8/docs/api/java/time/package-summary.html
Jaxb won't give any advantages in acis. Writing new app with jaxb usage is cool, but rewriting to jaxb is pain in ass.
Xmls in l2j are no more than config anyway. Just key-value pairs.