script.js

const input = document.querySelector('input')
let date = new Date("May 28, 2021 00:00:00").getTime();

input.addEventListener('change', () => {
    let selectedDate = new Date(input.value);

    selectedDate.setMinutes(selectedDate.getTimezoneOffset());

    let formattedDate = selectedDate.toLocaleDateString('en-US', {
        year: 'numeric',
        month: 'long',
        day: 'numeric'
    });

    document.querySelector('h2').textContent = formattedDate

    date = selectedDate.getTime();
})

const countdown = () => {
    const now = new Date().getTime();
    const gap = date - now;

    const second = 1000;
    const minute = second * 60;
    const hour = minute * 60;
    const day = hour * 24;

    const textDay = Math.floor(gap / day);
    const textHour = Math.floor((gap % day) / hour);
    const textMinute = Math.floor((gap % hour) / minute);
    const textSecond = Math.floor((gap % minute) / second);
    
    document.querySelector(".day").textContent = textDay;
    document.querySelector(".hour").textContent = textHour;
    document.querySelector(".minute").textContent = textMinute;
    document.querySelector(".second").textContent = textSecond;
}

setInterval(countdown, 1000);