code: def get_session_not_on_or_after(self):
          
          not_on_or_after = None
          authn_statement_nodes = self.__query_assertion('/saml:AuthnStatement[@SessionNotOnOrAfter]')
          if authn_statement_nodes:
              not_on_or_after = OneLogin_Saml2_Utils.parse_SAML_to_time(authn_statement_nodes[0].get('SessionNotOnOrAfter'))
          return not_on_or_after
docs: Gets the SessionNotOnOrAfter from the AuthnStatement
          Could be used to set the local session expiration
  
          :returns: The SessionNotOnOrAfter value
          :rtype: time|None