simpleTimer.js

(() => {
  const timerElement = document.getElementById('timer')
  const startTime = new Date().getTime()

  function pad(num, size) {
    let s = "0000" + String(num)
    return s.substr(s.length - size)
  }

  function getTime() {
    const now = new Date().getTime()
    const distance = now - startTime

    return {
      hours: Math.floor((distance / (1000 * 60 * 60))),
      minutes: Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60)),
      seconds: Math.floor((distance % (1000 * 60)) / 1000)
    }
  }

  function update() {
    timerElement.innerHTML = `${getTime().hours}:${pad(getTime().minutes, 2)}:${pad(getTime().seconds, 2)}`
  }

  var clocktimer = setInterval(update, 1000)
})()