code: def when_starts(self, timeformat='unix'):
          
          start_coverage = min([item.get_reference_time() \
                                for item in self._forecast])
          return timeformatutils.timeformat(start_coverage, timeformat)
docs: 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