@staticmethod
def metadata_updated_on(item):
    """Extracts and coverts the update time from a Docker Hub item.

        The timestamp is extracted from 'fetched_on' field. This field
        is not part of the data provided by Docker Hub. It is added
        by this backend.

        :param item: item generated by the backend

        :returns: a UNIX timestamp
        """
    return item['fetched_on']
