public static function Tdbtt($tdb1, $tdb2, $dtr, &$tt1, &$tt2) {
    $dtrd;

    /* Result, safeguarding precision. */
    $dtrd = $dtr / DAYSEC;
    if ($tdb1 > $tdb2) {
      $tt1 = $tdb1;
      $tt2 = $tdb2 - $dtrd;
    }
    else {
      $tt1 = $tdb1 - $dtrd;
      $tt2 = $tdb2;
    }

    /* Status (always OK). */
    return 0;
  }