simple javascript chronometer

 <div class="container">
        <h1>Chronometer</h1>
        <div class="display" id="display">00:00:00</div>
        <div class="controls">
            <button class="start" id="startBtn">Start</button>
            <button class="stop" id="stopBtn" disabled>Stop</button>
            <button class="reset" id="resetBtn">Reset</button>
        </div>
    </div> 

   * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        body {
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
            min-height: 100vh;
            display: flex;
            justify-content: center;
            align-items: center;
            background: linear-gradient(135deg, #D1E231 0%, #a8b828 100%);
        }

        .container {
            background: white;
            padding: 3rem 4rem;
            border-radius: 20px;
            box-shadow: 0 20px 60px rgba(0, 0, 0, 0.2);
            text-align: center;
            min-width: 400px;
        }

        h1 {
            color: #333;
            font-size: 1.8rem;
            margin-bottom: 2rem;
            font-weight: 600;
        }

        .display {
            font-size: 4rem;
            font-weight: 300;
            color: #2c3e50;
            margin-bottom: 2.5rem;
            font-variant-numeric: tabular-nums;
            letter-spacing: 0.05em;
        }

        .controls {
            display: flex;
            gap: 1rem;
            justify-content: center;
        }

        button {
            padding: 0.9rem 2rem;
            font-size: 1rem;
            border: none;
            border-radius: 8px;
            cursor: pointer;
            font-weight: 500;
            transition: all 0.3s ease;
            text-transform: uppercase;
            letter-spacing: 0.05em;
        }

        .start {
            background: linear-gradient(135deg, #D1E231 0%, #a8b828 100%);
            color: #2c3e50;
        }

        .start:hover {
            transform: translateY(-2px);
            box-shadow: 0 5px 15px rgba(209, 226, 49, 0.4);
        }

        .stop {
            background: #e74c3c;
            color: white;
        }

        .stop:hover {
            background: #c0392b;
            transform: translateY(-2px);
            box-shadow: 0 5px 15px rgba(231, 76, 60, 0.4);
        }

        .reset {
            background: #95a5a6;
            color: white;
        }

        .reset:hover {
            background: #7f8c8d;
            transform: translateY(-2px);
            box-shadow: 0 5px 15px rgba(149, 165, 166, 0.4);
        }

        button:active {
            transform: translateY(0);
        }

        button:disabled {
            opacity: 0.5;
            cursor: not-allowed;
            transform: none;
        }

   let startTime = 0;
        let elapsedTime = 0;
        let timerInterval = null;

        const display = document.getElementById('display');
        const startBtn = document.getElementById('startBtn');
        const stopBtn = document.getElementById('stopBtn');
        const resetBtn = document.getElementById('resetBtn');

        function formatTime(ms) {
            const totalSeconds = Math.floor(ms / 1000);
            const hours = Math.floor(totalSeconds / 3600);
            const minutes = Math.floor((totalSeconds % 3600) / 60);
            const seconds = totalSeconds % 60;

            return `${String(hours).padStart(2, '0')}:${String(minutes).padStart(2, '0')}:${String(seconds).padStart(2, '0')}`;
        }

        function updateDisplay() {
            const currentTime = Date.now();
            elapsedTime = currentTime - startTime;
            display.textContent = formatTime(elapsedTime);
        }

        function start() {
            startTime = Date.now() - elapsedTime;
            timerInterval = setInterval(updateDisplay, 100);
            startBtn.disabled = true;
            stopBtn.disabled = false;
        }

        function stop() {
            clearInterval(timerInterval);
            startBtn.disabled = false;
            stopBtn.disabled = true;
        }

        function reset() {
            clearInterval(timerInterval);
            startTime = 0;
            elapsedTime = 0;
            display.textContent = '00:00:00';
            startBtn.disabled = false;
            stopBtn.disabled = true;
        }

        startBtn.addEventListener('click', start);
        stopBtn.addEventListener('click', stop);
        resetBtn.addEventListener('click', reset);