MediaWiki:Common.js

/* Any JavaScript here will be loaded for all users on every page load. */
        const countDownDate = 'January 21 2021 05:00:00 GMT+0000';

        function getTimeRemaining(endtime) {
            const total = Date.parse(endtime) - Date.parse(new Date());
            const seconds = Math.floor((total / 1000) % 60);
            const minutes = Math.floor((total / 1000 / 60) % 60);
            const hours = Math.floor((total / (1000 * 60 * 60)) % 24);
            const days = Math.floor(total / (1000 * 60 * 60 * 24));
            return {
            	total: total, 
            	days: days, 
            	hours: hours,  
            	minutes: minutes, 
            	seconds: seconds
            };
        }

        function initializeClock(id, endtime) {
            const clock = document.getElementById(id);
            const timeinterval = setInterval(() => {
                const t = getTimeRemaining(endtime);
                clock.innerHTML = t.days + "d " + t.hours + "h " + t.minutes + "m " + t.seconds + "s ";
                if (t.total <= 0) {
                    clearInterval(timeinterval);
                }
            }, 1000);
        }

        initializeClock('launch_timer', countDownDate);