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

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

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