为什么setInterval可以实现效果，但用setTimeout,秒数没有变？

<!DOCTYPE HTML> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>团购——限时抢</title> <link rel="stylesheet" href="style.css" /> </head> <body> <div class="content3"> <div class="time">还剩 <span id="LeftTime"></span></div> </div> <script> function FreshTime() { var endtime=new Date("2017/5/15,12:20:12");//结束时间 var nowtime = new Date();//当前时间 var lefttime=parseInt((endtime.getTime()-nowtime.getTime())/1000); d=parseInt(lefttime/(60*60*24)); h=parseInt(lefttime/(60*60)%24); m=parseInt(lefttime/60%60); s=parseInt(lefttime%60); document.getElementById("LeftTime").innerHTML=d+"天"+h+"小时"+m+"分"+s+"秒"; if(lefttime<=0){ document.getElementById("LeftTime").innerHTML="团购已结束"; clearInterval(sh); } } FreshTime() var sh; //sh=setInterval(FreshTime,500); sh=setTimeout(FreshTime,500); </script> </body> </html>

<!DOCTYPE HTML> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>团购——限时抢</title> <link rel="stylesheet" href="style.css" /> </head> <body> <div class="content3"> <div class="time">还剩 <span id="LeftTime"></span></div> </div> <script> function FreshTime() { var endtime=new Date("2017/5/15,12:20:12");//结束时间 var nowtime = new Date();//当前时间 var lefttime=parseInt((endtime.getTime()-nowtime.getTime())/1000); d=parseInt(lefttime/(60*60*24)); h=parseInt(lefttime/(60*60)%24); m=parseInt(lefttime/60%60); s=parseInt(lefttime%60); document.getElementById("LeftTime").innerHTML=d+"天"+h+"小时"+m+"分"+s+"秒"; if(lefttime<=0){ document.getElementById("LeftTime").innerHTML="团购已结束"; clearTimeout(sh); }else{ setTimeout(FreshTime,500); } } FreshTime() var sh; sh=setTimeout(FreshTime,500); </script> </body> </html>其实原理是一样的，多一个else判断，欢迎采纳

为什么setInterval可以实现效果，但用setTimeout,秒数没有变？

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