Dates are JavaScript objects, and _pack_models returns an empty object

IPython/html/static/widgets/js/widget.js

@@ -236,7 +236,8 b' define(["widgets/js/manager",'
                     packed.push(that._pack_models(sub_value));
                 });
                 return packed;
-
+            } else if (value instanceof Date) {
+                return value;
             } else if (value instanceof Object) {
                 packed = {};
                 _.each(value, function(sub_value, key) {