def METHOD_NAME(self, modified):
    """
    Sets modified if it's less than the existing, otherwise sets None if not a datetime
    :param modified: A datetime or None
    """
    if not isinstance(modified, datetime):
        return self.set_modified(None)
    current_modified = self.get_modified()
    if current_modified and current_modified > modified:
        return self.set_modified(modified)