Immutable java.lang.DateTime class
java.util.Date is ugly. It is unsafe for use in maps and sets. It's unlike other simple wrapper classes such as String, Integer. I propose the introduction of a class java.lang.DateTime as an immutable object. For the new APIs around it, java.util.Date could extend java.lang.DateTime. This would provide for all new DateTime APIs to work with existing objects created using java.util.Date.