通过js实现倒计时

<script LANGUAGE="javascript"> 
startclock() 
var timerID = null; 
var timerRunning = false; 
function showtime() { 
Today = new Date(); 
var NowHour = Today.getHours(); 
var NowMinute = Today.getMinutes(); 
var NowMonth = Today.getMonth(); 
var NowDate = Today.getDate(); 
var NowYear = Today.getYear(); 
var NowSecond = Today.getSeconds(); 
if (NowYear <2000) 
NowYear=1900+NowYear; 
Today = null; 
Hourleft = 23 - NowHour 
Minuteleft = 59 - NowMinute 
Secondleft = 59 - NowSecond 
Yearleft = 2019 - NowYear 
Monthleft = 12 - NowMonth - 1 
Dateleft = 31 - NowDate 
if (Secondleft<0) 
{ 
Secondleft=60+Secondleft; 
Minuteleft=Minuteleft-1; 
} 
if (Minuteleft<0) 
{ 
Minuteleft=60+Minuteleft; 
Hourleft=Hourleft-1; 
} 
if (Hourleft<0) 
{ 
Hourleft=24+Hourleft; 
Dateleft=Dateleft-1; 
} 
if (Dateleft<0) 
{ 
Dateleft=31+Dateleft; 
Monthleft=Monthleft-1; 
} 
if (Monthleft<0) 
{ 
Monthleft=12+Monthleft; 
Yearleft=Yearleft-1; 
} 
Temp=Yearleft+'年, '+Monthleft+'月, '+Dateleft+'天, '+Hourleft+'小时, '+Minuteleft+'分, '+Secondleft+'秒' 
document.form1.left.value=Temp; //用于向form标签传递数据
timerID = setTimeout("showtime()",1000); 
timerRunning = true; 
} 
var timerID = null; 
var timerRunning = false; 
function stopclock () { 
if(timerRunning) 
clearTimeout(timerID); 
timerRunning = false; 
} 
function startclock () { 
stopclock(); 
showtime(); 
} 
// --> 
</script>

<form name="form1">   
<div align="center" align="center">   
<center>离2020年还有:<br>   
<input type="textarea" name="left" size="35" style="text-align: center">   
</center>   
</div>