Таймер обратного отсчета 5 минут

function getCookie(c_name){var i,x,y,ARRcookies=document.cookie.split(";");for(i=0;i<ARRcookies.length;i++){x=ARRcookies[i].substr(0,ARRcookies[i].indexOf("="));y=ARRcookies[i].substr(ARRcookies[i].indexOf("=")+1);x=x.replace(/^\s+|\s+$/g,"");if(x==c_name){return unescape(y);}}}function setCookie(c_name,value,exdays){var exdate=new Date();exdate.setDate(exdate.getDate()+exdays);var c_value=escape(value)+((exdays==null)?"":"; expires="+exdate.toUTCString());document.cookie=c_name+"="+c_value;}var str=getCookie('Big Day');if(!str){var BigDay=new Date();BigDay.setMinutes(BigDay.getMinutes()+1395);setCookie('Big Day',BigDay,3);}else{BigDay=new Date(str);}var today=new Date();var diff=BigDay-today;
var countdown4=Math.round(diff/1000)
var countdown3=Math.round(diff/1000)
function convert_to_time(secs){secs=parseInt(secs);
hh=secs/3600; 
hh=parseInt(hh); 
mmt=secs-(hh*3600);
 
mm=(mmt/60);
 
 
mm=parseInt(mm);
 
 
ss=mmt-(mm*60);
 
if(hh>23){dd=hh/24;
dd=parseInt(dd);hh=hh-(dd*24);}else{dd=0;}if(ss<10){ss="0"+ss;}if(mm<10){mm="0"+mm;}if(hh<10){hh="0"+hh;}if(dd==0){
 
    return("<span>"+hh+"</span>"+"<b>:</b>"+"<span>"+mm+"</span>"+"<b>:</b>"+"<span>"+ss+"</span>");
 
}else{if(dd>1){return("<span>"+hh+"</span>"+"<b>:</b>"+"<span>"+mm+"</span>"+"<b>:</b>"+"<span>"+ss+"</span>");}else{return("<span>"+hh+"</span>"+"<b>:</b>"+"<span>"+mm+"</span>"+"<b>:</b>"+"<span>"+ss+"</span>");}}}do_cd4=function(){if(countdown4<0){document.getElementById('cd4').innerHTML="Il tempo è finito!";}else{document.getElementById('cd4').innerHTML=convert_to_time(countdown4);setTimeout('do_cd4()',1000);}countdown4=countdown4-1;}
do_cd3=function(){if(countdown3<0){document.getElementById('cd3').innerHTML="Il tempo è finito!";}else{document.getElementById('cd3').innerHTML=convert_to_time(countdown3);setTimeout('do_cd3()',1000);}countdown3=countdown3-1;}
document.write("<div id='cd4'></div>\n");do_cd4();document.write("<div id='cd3'></div>\n");do_cd3();

<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script async defer>
var seconds = 5*60+1, h,m,s,t;
function countDown() {
    if (seconds > 0) {
            seconds --;
                h = seconds/3600 ^ 0,
                m = (seconds-h*3600)/60 ^ 0,
                s = seconds-h*3600-m*60,
                time = (m<10?"0"+m:m)+":"+(s<10?"0"+s:s);
            $("body").text(time);
    } 
    setTimeout(function() {
        countDown();
    }, 1000);
}
$(function() {
  countDown()
});
</script>