public static function Ut1tt($ut11, $ut12, $dt, &$tt1, &$tt2) {
    $dtd;

    /* Result, safeguarding precision. */
    $dtd = $dt / DAYSEC;
    if ($ut11 > $ut12) {
      $tt1 = $ut11;
      $tt2 = $ut12 + $dtd;
    }
    else {
      $tt1 = $ut11 + $dtd;
      $tt2 = $ut12;
    }

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