def _build_date_header_string(self, date_value):
		"""Gets the date_value (may be None, basestring, float or
		datetime.datetime instance) and returns a valid date string as per
		RFC 2822."""
		if isinstance(date_value, datetime):
			date_value = time.mktime(date_value.timetuple())
		if not isinstance(date_value, basestring):
			date_value = formatdate(date_value, localtime=True)
		# Encode it here to avoid this:
		# Date: =?utf-8?q?Sat=2C_01_Sep_2012_13=3A08=3A29_-0300?=
		return native(date_value)
Gets the date_value (may be None, basestring, float or
		datetime.datetime instance) and returns a valid date string as per
		RFC 2822.