Measure time after click

timer.js

document.addEventListener('click', () => {
    const dt = Date.now();
    const clockElement = document.createElement('div');
    Object.assign(clockElement.style, {
        position: 'absolute',
        top: 0,
        right: 0,
        background: '#FAA',
        'z-index': 9999,
        padding: 5
    });
    document.body.appendChild(clockElement);

    setInterval(() => {
        clockElement.innerText = Date.now() - dt;
    }, 50);
});