js简单倒计时实现代码

<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>