def when_starts(self, timeformat='unix'):
        """
        Returns the GMT time of the start of the forecast coverage, which is
        the time of the most ancient *Weather* item in the forecast

        :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``
            '*date* for ``datetime.datetime`` object instance
        :type timeformat: str
        :returns: a long or a str
        :raises: *ValueError* when invalid time format values are provided

        """
        start_coverage = min([item.get_reference_time() \
                              for item in self._forecast])
        return timeformatutils.timeformat(start_coverage, timeformat)
Returns the GMT time of the start of the forecast coverage, which is
        the time of the most ancient *Weather* item in the forecast

        :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``
            '*date* for ``datetime.datetime`` object instance
        :type timeformat: str
        :returns: a long or a str
        :raises: *ValueError* when invalid time format values are provided