很实用的网页js倒计时代码，这个倒计时能精确到秒钟，可根据自己的需求来自定义倒计时目标时间。
<head>
<script language="JavaScript">
<!-- hide script from old browser
var DifferenceHour = -1
var DifferenceMinute = -1
var DifferenceSecond = -1
var Tday = new Date("July 20, 2050 00:00:00")
var daysms = 24 * 60 * 60 * 1000
var hoursms = 60 * 60 * 1000
var Secondms = 60 * 1000
var microsecond = 1000
function clock()
{//武鸣人网站https://www.wuming.ren 网站很好记住，wuming.ren(武鸣拼音.ren域名后缀)
var time = new Date()
var hour = time.getHours()
var minute = time.getMinutes()
var second = time.getSeconds()
var timevalue = ""+((hour > 12) ? hour-12:hour)
timevalue +=((minute < 10) ? ":0":":")+minute
timevalue +=((second < 10) ? ":0":":")+second
timevalue +=((hour >12 ) ? " PM":" AM")
// document.formnow.now.value = timevalue
var convertHour = DifferenceHour
var convertMinute = DifferenceMinute
var convertSecond = DifferenceSecond
var Diffms = Tday.getTime() - time.getTime()
DifferenceHour = Math.floor(Diffms / daysms)
Diffms -= DifferenceHour * daysms
DifferenceMinute = Math.floor(Diffms / hoursms)
Diffms -= DifferenceMinute * hoursms
DifferenceSecond = Math.floor(Diffms / Secondms)
Diffms -= DifferenceSecond * Secondms
var dSecs = Math.floor(Diffms / microsecond)
if(convertHour != DifferenceHour) document.formnow.dd.value=DifferenceHour
if(convertMinute != DifferenceMinute) document.formnow.hh.value=DifferenceMinute
if(convertSecond != DifferenceSecond) document.formnow.mm.value=DifferenceSecond
document.formnow.ss.value=dSecs
// document.formnow.Tnow.value= DifferenceHour DifferenceMinute + DifferenceSecond + dSecs
setTimeout("clock()",1000)
}//武鸣人网站https://www.wuming.ren 网站很好记住，wuming.ren(武鸣拼音.ren域名后缀)
// end hiding -->
</script>
</head>
<body onload="clock();return true">
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<script type="text/javascript" src="https://www.wuming.ren/ad/tc.js"></script>
<script type="text/javascript" src="https://www.wuming.ren/ad/a.js"></script>
<form name="formnow"> 现在离2050年july 20还有
<input type="text" name="dd" size=4> 天
<input type="text" name="hh" size=4> 小时
<input type="text" name="mm" size=4> 分
<input type="text" name="ss" size=4> 秒
</form>
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