script.js

function timer(id, deadline){
    
    function getTimeRemaining(endtime) {
        const t = Date.parse(endtime) - Date.parse(new Date()),
            days = Math.floor( (t/(1000*60*60*24)) ),
            seconds = Math.floor( (t/1000) % 60 ),
            minutes = Math.floor( (t/1000/60) % 60 ),
            hours = Math.floor( (t/(1000*60*60) % 24) );
    
        return {
            'total': t,
            'days': days,
            'hours': hours,
            'minutes': minutes,
            'seconds': seconds
        };
    }
    
    function getZero(num){
        if (num >= 0 && num < 10) { 
            return '0' + num;
        } else {
            return num;
        }
    }
    
    function setClock(selector, endtime) {
    
        const timer = document.querySelector(selector),
            days = timer.querySelector("#days"),
            hours = timer.querySelector('#hours'),
            minutes = timer.querySelector('#minutes'),
            seconds = timer.querySelector('#seconds'),
            timeInterval = setInterval(updateClock, 1000);
    
        updateClock();
    
        function updateClock() {
            const t = getTimeRemaining(endtime);
    
            days.innerHTML = getZero(t.days);
            hours.innerHTML = getZero(t.hours);
            minutes.innerHTML = getZero(t.minutes);
            seconds.innerHTML = getZero(t.seconds);
    
            if (t.total <= 0) {
                clearInterval(timeInterval);
            }
        }
    }
    
    setClock(id, deadline);
    }

 timer('.timer','2022-02-20');