PHP Cookie为1天

On my wordpress blog i want to show a popup on posts. I installed a plugin but it shows the popup for every time. I tried to change the code but didnt work. I dont know php well. Can you help me? I want to show popup only once per day.

if ( $timer && !empty( $show_timer ) && $show_timer == 'yes' && !isset($_COOKIE['html_ad']))

                $html_ad .= '<div id="popups-bar" ' . $style_bar . '>

                                    <div id="popups-tit" class="popups">' . $this->get_interad( $ad_id, 'title' ) . '</div>

                                    <div id="popups-close">

                                              <div id="inter-mess" ' . $bor_style . '>

                                                    <span> ' . $tra_wait . ' </span>

                                                    <span data-seconds="' . $countdown . '" class="popups-kkcount-down"></span>

                                                    <span> ' . $tra_until . ' </span>

                                              </div>    

                                    </div>

                            </div>';





            $html_ad .= '<div id="popups-cnt" ' . $style_cont . '>

                                ' . $this->get_interad( $ad_id ) . '

                         </div>



              </div><!-- Popup Plugin-->' . "
";
            setcookie('html_ad', true,  time()+86400);