static __getSyncDownValue(modelValue) {
    const isDate = new IsDate();
    const hasError = isDate.execute(modelValue);
    return hasError ? undefined : modelValue;
  }
The LionCalendar shouldn't know anything about the modelValue;
it can't handle Unparseable dates, but does handle 'undefined'
@param {?} modelValue
@returns {Date | undefined} a 'guarded' modelValue