def METHOD_NAME(self, timeformat='unix'):
    """
    Returns the GMT time telling when the CO observation has been received
    from the OWM Weather API
    :param timeformat: the format for the time value. May be:
        '*unix*' (default) for UNIX time
        '*iso*' for ISO8601-formatted string in the format ``YYYY-MM-DD HH:MM:SS+00:00``
        '*date* for ``datetime.datetime`` object instance
    :type timeformat: str
    :returns: an int or a str
    :raises: ValueError when negative values are provided
    """
    return formatting.timeformat(self.rec_time, timeformat)