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>