code: def _build_date_header_string(self, date_value):
  		
  		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)
docs: Gets the date_value (may be None, basestring, float or
  		datetime.datetime instance) and returns a valid date string as per
  		RFC 2822.