code: def format_modified(self, modified, sep=" "):
          
          if modified is not None:
              return modified.strftime("%Y-%m-%d{0}%H:%M:%S.%fZ".format(sep))
          return u""
docs: Format modification date in UTC if it's not None.
          @param modified: modification date in UTC
          @ptype modified: datetime or None
          @return: formatted date or empty string
          @rtype: unicode