网页中实现一个计算当年还剩多少时间的倒数计时程序，要求网页上实时动态显示“××年还剩××天××时××分××秒”。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>倒计时</title>
</head>
<body>
<!--显示倒计时的div-->
<div id="showTime"></div>
<script>
//输入一个倒计时开始时间
var isTime = new Date('2018-06-7 00:00:00');
var isMillis = isTime.getTime() - new Date().getTime();
var showTime = document.getElementById('showTime');
if(isMillis > 0) {
//计算倒计时的函数
function datetime() {
isMillis = isTime.getTime() - new Date().getTime();
var tian = Math.floor(isMillis / 1000 / 60 / 60 / 24);
if(tian < 10) {
hours = '0' + hours;
}
var hours = Math.floor((isMillis - tian * 24 * 60 * 60 * 1000) / 1000 / 60 / 60);
if(hours < 10) {
hours = '0' + hours;
}
var minutes = Math.floor((isMillis - tian * 24 * 60 * 60 * 1000 - hours * 60 * 60 * 1000) / 1000 / 60);
if(minutes < 10) {
minutes = '0' + minutes;
}
var seconds = Math.floor((isMillis - tian * 24 * 60 * 60 * 1000 - hours * 60 * 60 * 1000 - minutes * 60 * 1000) / 1000);
if(seconds < 10) {
seconds = '0' + seconds;
}
showTime.innerHTML = tian + '天' + hours + '小时' + minutes + '分钟' + seconds + '秒';
}
datetime();
} else {
document.write('结束时间不正确！')
}
//每个一秒执行一次，实现倒计时的效果
setInterval(datetime, 1000);
</script>
</body>
</html>
