MediaWiki:Common.js

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function countdownToChristmas() {
    var now = new Date();
    var targetDate = new Date(now.getFullYear(), 11, 25); // Dec 25 of current year

    if (now > targetDate) {
        targetDate.setFullYear(targetDate.getFullYear() + 1);
    }

    function updateCountdown() {
        var now = new Date();
        var diff = targetDate - now;

        if (diff <= 0) {
            document.getElementById('xmas-timer').innerText = "Merry Christmas!";
            return;
        }

        var days = Math.floor(diff / (1000 * 60 * 60 * 24));
        var hours = Math.floor((diff / (1000 * 60 * 60)) % 24);
        var minutes = Math.floor((diff / (1000 * 60)) % 60);
        var seconds = Math.floor((diff / 1000) % 60);

        document.getElementById('xmas-timer').innerText =
            "Countdown to Christmas: " + days + "d " + hours + "h " + minutes + "m " + seconds + "s";
    }

    updateCountdown();
    setInterval(updateCountdown, 1000);
}

$(document).ready(countdownToChristmas);