本文实例讲述了js简单倒计时实现代码。分享给大家供大家参考，具体如下：
<div class="time">
距离活动结束还有<span id="day"></span>天
<span id="hours"></span>小时
<span id="min"></span>分
<span id="sec"></span>秒
</div>
<script type="text/javascript">
var day = document.getElementById("day");
var hours = document.getElementById("hours");
var min = document.getElementById("min");
var sec = document.getElementById("sec");
var DifferenceHour = -1;
var DifferenceMinute = -1;
var DifferenceSecond = -1;
//var Tday = new Date("07 20,2016 10:00:00") //**倒计时时间点-注意格式
var Tday = new Date("07/20/2016 10:00:00"); //IE:月/日/年
var daysms = 24 * 60 * 60 * 1000;
var hoursms = 60 * 60 * 1000;
var Secondms = 60 * 1000;
var microsecond = 1000;
function clock () {
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) {
day.innerHTML = DifferenceHour;
}
if (convertMinute != DifferenceMinute) {
hours.innerHTML = DifferenceMinute;
}
if (convertSecond != DifferenceSecond) {
min.innerHTML = DifferenceSecond;
}
sec.innerHTML = dSecs;
// document.formnow.Tnow.value= DifferenceHour DifferenceMinute + DifferenceSecond + dSecs
setTimeout("clock()", 1000);
}
clock();
</script>
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希望本文所述对大家JavaScript程序设计有所帮助。