code: static Map<String, Object> toWire(Map<String, Object> json) {
          return (Map<String, Object>) ObjectIterator.iterate(json, new Closure<ObjectIterator.IterateBean, Object>() {
              @Override
              public Object call(ObjectIterator.IterateBean i) {
                  try {
                      if (i.getValue() instanceof Date) {
                          SimpleDateFormat sdf = new SimpleDateFormat("yyy-MM-dd HH:mm:ss.SSS");
                          //return "/Date(" + ((Date) i.getValue()).getTime() + ")/";
                          return "/Date(" + sdf.format((Date) i.getValue()) + ")/";
                      }
                  } catch (Exception e) {
                  }
                  return i.getValue();
              }
          });
      }
docs: Format Java Classes into strings version
  
  @param json
  @return