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HTML

              
                  <div class="timer">
    <div class="timer__controls">
      <button data-time="20" class="timer__button">20 Secs</button>
      <button data-time="300" class="timer__button">Work 5</button>
      <button data-time="900" class="timer__button">Quick 15</button>
      <button data-time="1200" class="timer__button">Snack 20</button>
      <button data-time="3600" class="timer__button">Lunch Break</button>
      <form name="customForm" id="custom">
        <input type="text" name="minutes" placeholder="Enter Minutes">
      </form>
    </div>
    <div class="display">
      <h1 class="display__time-left"></h1>
      <p class="display__end-time"></p>
    </div>
  </div>

              
            
!

CSS

              
                html {
  box-sizing: border-box;
  font-size: 10px;
  background: #8E24AA;
  background: linear-gradient(45deg,  #42a5f5 0%,#478ed1 50%,#0d47a1 100%);
}

*, *:before, *:after {
  box-sizing: inherit;
}

body {
  margin: 0;
  text-align: center;
  font-family: 'Inconsolata', monospace;
}

.display__time-left {
  font-weight: 100;
  font-size: 20rem;
  margin: 0;
  color: white;
  text-shadow: 4px 4px 0 rgba(0,0,0,0.05);
}

.timer {
  display: flex;
  min-height: 100vh;
  flex-direction: column;
}

.timer__controls {
  display: flex;
}

.timer__controls > * {
  flex: 1;
}

.timer__controls form {
  flex: 1;
  display: flex;
}

.timer__controls input {
  flex: 1;
  border: 0;
  padding: 2rem;
}

.timer__button {
  background: none;
  border: 0;
  cursor: pointer;
  color: white;
  font-size: 2rem;
  text-transform: uppercase;
  background: rgba(0,0,0,0.1);
  border-bottom: 3px solid rgba(0,0,0,0.2);
  border-right: 1px solid rgba(0,0,0,0.2);
  padding: 1rem;
  font-family: 'Inconsolata', monospace;
}

.timer__button:hover,
.timer__button:focus {
  background: rgba(0,0,0,0.2);
  outline: 0;
}

.display {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}

.display__end-time {
  font-size: 4rem;
  color: white;
}

              
            
!

JS

              
                let countdown
const timerDisplay = document.querySelector('.display__time-left')
const endTime = document.querySelector('.display__end-time')
const buttons = document.querySelectorAll('[data-time]')

function timer(seconds){
  // 開始另一個 timer 之前先把之前的 setInterval 停掉
  clearInterval(countdown)
  // const now = (new Date()).getTime()
  // 上面這種寫法也可以
  const now = Date.now() // mili seconds 的形式
  const then = now + seconds * 1000
  displayEndTime(then)
  displayTimeLeft(seconds)

  countdown = setInterval(function(){
    const secondsLeft = Math.round((then - Date.now()) / 1000)
    if(secondsLeft < 0) {
      clearInterval(countdown)
      return
    }
    displayTimeLeft(secondsLeft)
  }, 1000)
}

function displayTimeLeft(seconds){
  const minutes = Math.floor(seconds / 60)
  const remainSeconds = seconds % 60
  const display = `${minutes}:${remainSeconds < 10 ? '0' : ''}${remainSeconds}`
  document.title = display
  timerDisplay.textContent = display
}

function displayEndTime(timestamp){
  const end = new Date(timestamp)
  const hours = end.getHours()
  const minutes = end.getMinutes()
  endTime.textContent = `Be Back At ${hours}:${minutes < 10 ? '0' : ''}${minutes}`
}

function startTimer(){
  const period = this.dataset.time
  timer(period)
}

buttons.forEach(button => button.addEventListener('click', startTimer))

document.customForm.addEventListener('submit', function(e){
  e.preventDefault()
  const mins = this.minutes.value
  timer(mins * 60)
  this.reset()
})
              
            
!
999px

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