<script type="text/javascript" src="js/jquery.min.js"></script>
<script type="text/javascript" src="js/jquery.downCount.js"></script>
<div class="time2">
    <span class="days">00</span>:<span class="hours">00</span>:<span class="minutes">00</span>:<span class="seconds">00</span>
</div>
<script class="source" type="text/javascript">
    $(‘.time2‘).downCount({
        date: ‘12/24/2020 12:00:00‘,
        offset: 8//时区东8区
    }, function () {
        alert(‘倒计时结束!‘);
    });
</script>

jquery库 传送门——> https://www.cnblogs.com/comedy/p/12935841.html

//jquery.downCount.js，直接引入

(function($){$.fn.downCount=function(options,callback){var settings=$.extend({date:null,offset:null},options);if(!settings.date){$.error(‘Date is not defined.‘);}
if(!Date.parse(settings.date)){$.error(‘Incorrect date format, it should look like this, 12/24/2012 12:00:00.‘);}
var container=this;var currentDate=function(){var date=new Date();var utc=date.getTime()+(date.getTimezoneOffset()*60000);var new_date=new Date(utc+(3600000*settings.offset))
return new_date;};function countdown(){var target_date=new Date(settings.date),current_date=currentDate();var difference=target_date-current_date;if(difference<0){clearInterval(interval);if(callback&&typeof callback===‘function‘)callback();return;}
var _second=1000,_minute=_second*60,_hour=_minute*60,_day=_hour*24;var days=Math.floor(difference/_day),hours=Math.floor((difference%_day)/_hour),minutes=Math.floor((difference%_hour)/_minute),seconds=Math.floor((difference%_minute)/_second);days=(String(days).length>=2)?days:‘0‘+days;hours=(String(hours).length>=2)?hours:‘0‘+hours;minutes=(String(minutes).length>=2)?minutes:‘0‘+minutes;seconds=(String(seconds).length>=2)?seconds:‘0‘+seconds;var ref_days=(days===1)?‘day‘:‘days‘,ref_hours=(hours===1)?‘hour‘:‘hours‘,ref_minutes=(minutes===1)?‘minute‘:‘minutes‘,ref_seconds=(seconds===1)?‘second‘:‘seconds‘;container.find(‘.days‘).text(days);container.find(‘.hours‘).text(hours);container.find(‘.minutes‘).text(minutes);container.find(‘.seconds‘).text(seconds);container.find(‘.days_ref‘).text(ref_days);container.find(‘.hours_ref‘).text(ref_hours);container.find(‘.minutes_ref‘).text(ref_minutes);container.find(‘.seconds_ref‘).text(ref_seconds);};var interval=setInterval(countdown,1000);};})(jQuery);