Javascript实现倒计时和根据某时间开始计算时间

1、根据某时间计算经过了多长时间

JavaScript 代码

<script type="text/javascript">

        var time_start = new Date('2018','7','15');//开始时间，注意！！！如果从8月15开始，月份就是7月15
        var clock_start = time_start.getTime();


        //计算时间差
        function get_time_spent() {
            var time_now = new Date();
            return ((time_now.getTime() - clock_start));
        }

        function get_times() {
            var i_total_secs = Math.round(get_time_spent());

            //计算出相差天数
            var days = Math.floor(i_total_secs / (24 * 3600 * 1000))

            //计算出小时数
            var leave1 = i_total_secs % (24 * 3600 * 1000)    //计算天数后剩余的毫秒数
            var hours = Math.floor(leave1 / (3600 * 1000))
            //计算相差分钟数
            var leave2 = leave1 % (3600 * 1000)        //计算小时数后剩余的毫秒数
            var minutes = Math.floor(leave2 / (60 * 1000))
            //计算相差秒数
            var leave3 = leave2 % (60 * 1000)      //计算分钟数后剩余的毫秒数
            var seconds = Math.round(leave3 / 1000)

            if (seconds.toString().length == 1) {
                seconds = '0' + seconds;
            }
            console.log(days + '天' + hours + '个小时' + minutes + '分钟' + seconds + '秒');
            document.getElementById('timer').innerHTML="<div>" + days + " <span>天</span></div>" +
                "<div>" + hours + " <span>小时</span></div>" +
                "<div>" + minutes + " <span>分钟</span></div>" +
                "<div>" + seconds + " <span>秒</span></div>"
            window.setTimeout('show_secs()', 1000);   //每隔1s刷新一次input里的值
        }
    </script>

HTML代码

<body onload=" window.setTimeout('get_times()',1);">
<div id="timer">
   <div>296 <span>天</span></div>
   <div>23 <span>小时</span></div>
   <div>32 <span>分钟</span></div>
   <div>01 <span>秒</span></div>
</div>
</body>

2、倒计时

JavaScript代码

<script type="text/javascript">
    //获取截至时间
    const endTime = new Date("2019/10/21 00:00:00").getTime();
    let timer = setInterval(function() {
        const today = new Date().getTime();
        // 获取时间差
        const timer = endTime - today;
        // 获取天数、月数、分钟数、秒数
        let days = Math.floor(timer / (1000 * 60 * 60 * 24));
        let hours = Math.floor((timer % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
        let minutes = Math.floor((timer % (1000 * 60 * 60)) / (1000 * 60));
        let seconds = Math.floor((timer % (1000 * 60)) / 1000);

        if(days <=0){
            document.getElementById('days').innerHTML=0;
            document.getElementById('hours').innerHTML=0;
            document.getElementById('minutes').innerHTML=0;
            document.getElementById('seconds').innerHTML=0;
        }else{
            document.getElementById('days').innerHTML=days;
            document.getElementById('hours').innerHTML=hours;
            document.getElementById('minutes').innerHTML=minutes;
            document.getElementById('seconds').innerHTML=seconds;
        }

    }, 1000);
</script>

HTML代码

<div>
 <span id="days"></span>天
 <span id="hours"></span>小时
 <span id="minutes"></span>分
 <span id="seconds"></span>秒
</div>

Javascript实现倒计时和根据某时间开始计算时间

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