index.js

const display = document.getElementById('display');
let timer = null;
let startTime = 0;
let elapsedTime = 0;
let isRunning = false;

function start() {
	if (!isRunning) {
		startTime = Date.now() - elapsedTime;
		timer = setInterval(update, 10);
		isRunning = true;
	}
}

function stop() {
	if (isRunning) {
		clearInterval(timer);
		elapsedTime = Date.now() - startTime;
		isRunning = false;
	}
}

function reset() {
	clearInterval(timer);
	startTime = 0;
	elapsedTime = 0;
	isRunning = false;
	display.textContent = '00:00:00:00';
}

function update() {
	const currentTime = Date.now();
	elapsedTime = currentTime - startTime;

	let hours = Math.floor(elapsedTime / (1000 * 60 * 60));
	let minutes = Math.floor((elapsedTime / (1000 * 60)) % 60);
	let seconds = Math.floor((elapsedTime / 1000) % 60);
	let miliseconds = Math.floor((elapsedTime % 1000) / 10);

	hours = String(hours).padStart(2, '0');
	minutes = String(minutes).padStart(2, '0');
	seconds = String(seconds).padStart(2, '0');
	miliseconds = String(miliseconds).padStart(2, '0');

	display.textContent = `${hours}:${minutes}:${seconds}:${miliseconds}`;
}