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：

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();

希望本文所述对大家JavaScript程序设计有所帮助。