js cookie 頁面倒數計時

瘋了啦 寫了一篇沒有儲存
需求:頁面倒數計時 只從第一次加購開始
公共方法
cookie的設定 獲取
function getCookie(c_name)
{

if (document.cookie.length>0)
{
    c_start=document.cookie.indexOf(c_name + "=");
    if (c_start!=-1)
    {
        c_start=c_start + c_name.length+1;
        c_end=document.cookie.indexOf(";",c_start);
        if (c_end==-1) c_end=document.cookie.length;;
        return unescape(document.cookie.substring(c_start,c_end))
    }
}
return ""

}
function setCookie(cname,cvalue,exdays) {

var d = new Date();
d.setTime(d.getTime() + (exdays*60*1000));
var expires = "expires=" + d.toGMTString();
document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";

}

實現步驟
點選加購 -> 記錄點選當前時間 ->設定個cookie
在點選頁面：
function payment() {

    var n = getCookie(`now_time`);
    if(!n){
        var nowtime = new Date();
        nowtime= nowtime.getTime();
        setCookie(`now_time`,nowtime,5);
    }
   
}

在支付頁面呼叫方法：
now：當前的時間戳
function PaymentCountdown(now){

  var now = now;
  var w = getCookie(`now_time`);
  if(w){
      var timeout =setInterval(function(){
          var dateTime = new Date();
          dateTime = dateTime.getTime();
          var diff = dateTime - now;
          var alltime = 5*60*1000;
          var c = alltime-diff;
          if(c>=0){
              var m = Math.floor(c/1000/60%60);
              var s = Math.floor(c/1000%60);
              var str =  "<span>"+m+"</span>:<span>"+s+"</span>";
              jQuery("#payment_time").html(str);
          }else if(c<0){
              clearInterval(timeout);
              jQuery("#payment_words").hide();
              jQuery("#payment_end").show();
          }
      }, 1000);

  }

};