<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>
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;
}
const timerDisplay = document.querySelector(".display__time-left");
const endTime = document.querySelector(".display__end-time");
const buttons = document.querySelectorAll("[data-time]");

let countdown; // this is the ret value if setInterval

function timer(seconds) {
  // clear any existing timers
  clearInterval(countdown);

  const now = Date.now();
  // convert seconds to mili seconds
  const then = now + seconds * 1000;
  displayTimeLeft(seconds);
  displayEndTime(then);

  countdown = setInterval(() => {
    const secondsLeft = Math.round((then - Date.now()) / 1000);
    // check if we should stop the countdown
    if (secondsLeft < 0) clearInterval(countdown);
    else
      // else display secondsLeft
      displayTimeLeft(secondsLeft);
  }, 1000);
}

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

function displayEndTime(timestamp) {
  const end = new Date(timestamp);
  const hour = end.getHours();
  const adjustedHour = hour > 12 ? hour - 12 : hour;
  const minutes = end.getMinutes();
  const adjustedMinutes = `${minutes < 10 ? "0" : ""}${minutes}`;
  endTime.textContent = `Be Back At ${adjustedHour}:${adjustedMinutes}`;
}

function startTimer() {
  const seconds = parseInt(this.dataset.time);
  timer(seconds);
}

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();
});

External CSS

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External JavaScript

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