Grant long lasting token for the disclosure app

what about:
let $token := collection("Tokens")[$$."expiration-date" eq xs:dateTime("2016-09-12T22:17:23.851315Z")
                                                     and $$."user-id" eq $user."_id"]
return 
  if(empty($token))
  then session:start($user, xs:dateTime("2016-09-12T22:17:23.851315Z"));
  else ();