网站底部引入运行时间代码

<center>
网站已运行:<span id="run_time" style="color:white"></span>
</center>
        <script>
            function runTime() {
                var d = new Date(), str = '';
                BirthDay = new Date("March 06,2020");
                today = new Date();
                timeold = (today.getTime() - BirthDay.getTime());
                sectimeold = timeold / 1000
                secondsold = Math.floor(sectimeold);
                msPerDay = 24 * 60 * 60 * 1000
                msPerYear = 365 * 24 * 60 * 60 * 1000
                e_daysold = timeold / msPerDay
                e_yearsold = timeold / msPerYear
                daysold = Math.floor(e_daysold);
                yearsold = Math.floor(e_yearsold);
                //str = yearsold + "年";
                str += daysold + "天";
                str += d.getHours() + '时';
                str += d.getMinutes() + '分';
                str += d.getSeconds() + '秒';
                return str;
            }
            setInterval(function () { $('#run_time').html(runTime()) }, 1000);
        </script>