Responsive landing page, with JS UI elements

What are you most proud of, and what would you do differently next time?

I quite like how I managed to get the countdown timer to reset its date to 3 months in future no matter what the date is it will always be 3 months. Here's the script on how I managed to get it working:

/* ------------------------------- */
/* Countdown Timer                 */
/* ------------------------------- */

const today = new Date();
const targetDate = today.getTime() + 90 * 24 * 60 * 60 * 1000;

// Define month names before using them
const monthNames = [
    'Jan',
    'Feb',
    'Mar',
    'Apr',
    'May',
    'Jun',
    'Jul',
    'Aug',
    'Sep',
    'Oct',
    'Nov',
    'Dec',
];

// Cache countdown element references
const countdownHeading = document.querySelector('.countdown h2 span');
const countdownDay = document.getElementById('countdown-day');
const countdownHrs = document.getElementById('countdown-hrs');
const countdownMin = document.getElementById('countdown-min');
const countdownSec = document.getElementById('countdown-sec');

// Display the launch date in the countdown's 
countdownHeading.textContent = formatDate(new Date(targetDate));

// Update the countdown every second
const countdownInterval = setInterval(updateCountdown, 1000);

function updateCountdown() {
    const currentTime = Date.now(); // use Date.now() for efficiency
    const timeRemaining = targetDate - currentTime;

    if (timeRemaining ` look based off of the graphical mockup. This was achieved obviously with CSS and JS. While it is not ideal, I'm sure it could be wired up to be a functional component.