function displayTime() {
   let date = new Date();
   let time = date.toLocaleTimeString();
   document.getElementById('demo').textContent = time;
}

const createClock = setInterval(displayTime, 1000);

const myInterval = setInterval(myFunction, 2000);

clearInterval(myInterval);

• You probably ought to disable the start button after pressing it once, and enable it again after you've stopped/reset it. Otherwise multiple presses of the start button will apply multiple setInterval()s to the clock, leading to wrong behavior.