js 倒计时功能，获取当前时间的年月日，时分秒

一、实现当前时间到指定截止时间的倒计时功能

<html>
    <head>
        <title>TEST</title>
    </head>
    <body>
        <script>
            //获取当前时间距离截止时间的倒计时
            //参数为截止时间
            var leftTimer = function(year, month, day, hour, minute, second){
                var leftTime = (new Date(year, month-1, day, hour, minute, second)) - (new Date());//计算剩余毫秒数
                var days = parseInt(leftTime / 1000 / 60 / 60 / 24, 10);//计算剩余天数
                var hours = parseInt(leftTime / 1000 / 60 / 60 % 24, 10);//计算剩余小时数
                var minutes = parseInt(leftTime / 1000 / 60 % 60, 10);//计算剩分钟数
                var seconds = parseInt(leftTime / 1000 % 60, 10);//计算剩余秒数

                days = checkTime(days).toString();
                hours = checkTime(hours).toString();
                minutes = checkTime(minutes).toString();
                seconds = checkTime(seconds).toString();
                return days + ' : ' + hours + ' : ' + minutes + ' : ' + seconds
            }
            var checkTime = function(i){
                if(i < 10){
                    i = "0" + i;
                }
                return i;
            }
            setInterval(function() {
                document.getElementById('testId').innerHTML = leftTimer(2018, 12, 31, 11, 0, 0)
            }, 1000)
        </script>

        <div id="testId"> </div>

    </body>
</html>

二、获取当前的年月日时分秒

var currTime = new Date();             //获取当前时间的毫秒数
var year = currTime.getFullYear();     //获取当前时间的年份
var month = currTime.getMonth() + 1;   //获取当前时间的月份，月份从0开始，所以需要加一
var day = currTime.getDate();          //获取当前时间的日期，getDay()可以获取星期几
var hour = currTime.getHours();        //获取当前时间的小时数
var minute = currTime.getMinutes();    //获取当前时间的分钟数
var second = currTime.getSeconds();    //获取当前时间的秒数

三、毫秒数转成年月日时分秒

//毫秒数转换成时间
var getCurrentTime = function(milliseconds){
    var myDate = new Date(milliseconds);
    var year = myDate.getFullYear();
    var month = myDate.getMonth() + 1;
    var day = myDate.getDate()
    var hour = myDate.getHours();
    var minute = myDate.getMinutes();
    var second = myDate.getSeconds();

    month = checkTime(month).toString();
    day = checkTime(day).toString();
    hour = checkTime(hour).toString();
    minute = checkTime(minute).toString();
    second = checkTime(second).toString();

    return year+"-"+month+"-"+day+" "+hour+":"+minute+":"+second;
}
// 只有一位数字时添加“0”
var checkTime = function(i){
    if(i < 10){
        i = "0" + i;
    }
    return i;
}