public function shift($_time, $_startIndex = null, $_endIndex = null)
    {
        if (!is_int($_time)) {
            return false;
        }
        if ($_time == 0) {
            return true;
        }

        if (null === $_startIndex) {
            $_startIndex = 0;
        }
        if (null === $_endIndex) {
            $_endIndex = $this->getCuesCount() - 1;
        }

        $startCue = $this->getCue($_startIndex);
        $endCue   = $this->getCue($_endIndex);

        //check subtitles do exist
        if (!$startCue || !$endCue) {
            return false;
        }

        for ($i = $_startIndex; $i <= $_endIndex; $i++) {
                $cue = $this->getCue($i);
                $cue->shift($_time);
        }

        return true;
    }