Here is a dateinput polymer filter to convert its value to a DateTime.

DateInputFilter.dart

class DateTimeFromDateInput extends Transformer<String,DateTime>{
  String forward(DateTime d){
    String s = d.toString();
    return s.substring(0,s.lastIndexOf(' '));
  }

  DateTime reverse(String s) {
    try{
      return DateTime.parse(s);
    }on Exception {
      return null;
    }
  }
}