Countdown timer

<main class="container">
  <div class="progress"></div>
  <div class="counter">1</div>
  <button class="button">Toggle</button>
</main>

<script src="https://rawgit.com/lmgonzalves/momentum-slider/master/dist/momentum-slider.min.js"></script>

:root {
  --progress: 100;
}

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

body {
  overflow: hidden;
}
body:before {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  border-left: 50vw solid #FBD786; /* Градиент */;
  border-right: 50vw solid #FBD786;
  border-top: 50vh solid #C6FFDD;
  border-bottom: 50vh solid #C6FFDD;
  z-index: -1;
}

.container {
  position: absolute;
  left: 50%;
  top: 50%;
  width: 340px;
  height: 540px;
  background-color: #354051;
  box-shadow: 0 0 100px rgba(0, 0, 0, 0.5);
  transform: translate(-50%, -50%);
  overflow: hidden;
}
.container:before {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 25px;
  background-color: rgba(0, 0, 0, 0.2);
  z-index: 3;
}

.progress {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  z-index: 1;
  pointer-events: none;
}
.progress:before {
  content: "";
  position: absolute;
  left: calc(50% - 30px);
  bottom: 50px;
  width: 60px;
  height: 60px;
  background-color: #0c7702;
  border-radius: 100%;
  transform: scale(1);
  transition: 0.8s transform;
}
.progress:after {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0, 0, 0, 0.1);
  box-shadow: 0 50px 0 50px rgba(0, 0, 0, 0.1);
  transform: translateY(calc(var(--progress) * 1%));
  transition: 0.95s transform cubic-bezier(0.175, 0.885, 0.32, 1.275);
}

.counter {
  display: flex;
  align-items: center;
  justify-content: center;
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  font-size: 100px;
  font-family: "Roboto Mono", monospace;
  color: #FFFFFF;
  pointer-events: none;
  opacity: 0;
  z-index: 3;
  transition: 0.5s opacity;
}

.button {
  position: absolute;
  left: calc(50% - 30px);
  bottom: 50px;
  width: 60px;
  height: 60px;
  padding: 0;
  color: transparent;
  background-color: transparent;
  outline: none;
  border: none;
  border-radius: 100%;
  text-indent: -999px;
  z-index: 2;
  cursor: pointer;
}
.button:before {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  border-radius: 100%;
  background-color: transparent;
  transform: scale(0);
  transition: 0.5s 0.5s transform, 0.5s 0.5s background-color;
}
.button:after {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  border-radius: 100%;
  opacity: 0;
  box-shadow: inset 0 0 0 20px white;
  transition: 0s;
  pointer-events: none;
  z-index: -1;
}

.container--running .progress:before {
  transform: scale(17);
  transition: 0.8s transform cubic-bezier(0.6, -0.28, 0.735, 0.045);
}
.container--running .counter {
  opacity: 1;
  transition: 1s opacity;
}
.container--running .button:before {
  background-color: #354051;
  transform: scale(1);
  transition: 0.5s 0.5s transform, 0.5s 0.5s background-color;
}
.container--running .button:after {
  opacity: 1;
  transform: scale(2.5);
  box-shadow: inset 0 0 0 0 rgba(255, 255, 255, 0);
  transition: all 0.8s, opacity 0.3s;
}

.ms-container {
  position: relative;
  top: 50%;
  width: 340px;
  max-width: 100%;
  margin: 0 auto;
  overflow: hidden;
  transform: translateY(-50%);
}
.ms-container:before, .ms-container:after {
  content: "";
  position: absolute;
  top: 0;
  width: 80px;
  height: 100%;
  z-index: 1;
  pointer-events: none;
}
.ms-container:before {
  left: 0;
  background-image: linear-gradient(to right, #354051 25%, rgba(53, 64, 81, 0));
}
.ms-container:after {
  right: 0;
  background-image: linear-gradient(to left, #354051 25%, rgba(53, 64, 81, 0));
}

.ms-track {
  position: relative;
  left: calc(50% - 62.5px);
  white-space: nowrap;
  font-size: 0;
  list-style: none;
  padding: 0;
  margin: 0;
  will-change: transform;
}

.ms-slide {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 125px;
  height: 540px;
  font-size: 100px;
  font-family: "Roboto Mono", monospace;
  color: #FFFFFF;
  cursor: default;
  -webkit-user-select: none;
     -moz-user-select: none;
      -ms-user-select: none;
          user-select: none;
  will-change: transform;
}

(function() {

  // Variables to use later
  var root = document.documentElement;
  var container = document.querySelector('.container');
  var button = document.querySelector('.button');
  var counter = document.querySelector('.counter');
  var running = false;
  var timer = null;
  var seconds = 0;
  var secondsInitial = 0;

  // Initializing the slider
  var ms = new MomentumSlider({
      el: container,  // HTML element to append the slider
      range: [1, 60], // Generar los elementos del slider usando el rango de números definido
      loop: 2,        // Hacer infinito el slider, añadiendo 2 elementos extra en cada extremo
      style: {
          // Estilos para interpolar
          // El primer valor corresponde a los elementos adyacentes
          // El segundo valor corresponde al elemento actual
          transform: [{scale: [0.4, 1]}],
          opacity: [0.3, 1]
      }
  });

  // Simple toggle functionality
  button.addEventListener('click', function () {
      if (running) {
          stop();
      } else {
          start();
      }
      running = !running;
  });

  // Start the countdown
  function start() {
      // Disable the slider during countdown
      ms.disable();
      // Get current slide index, and set initial values
      seconds = ms.getCurrentIndex() + 1;
      counter.innerText = secondsInitial = seconds;
      root.style.setProperty('--progress', 0);
      // Add class to trigger CSS transitions for `running` state
      container.classList.add('container--running');
      // Set interval to update the component every second
      timer = setInterval(function () {
          // Update values
          counter.innerText = --seconds;
          root.style.setProperty('--progress', (secondsInitial - seconds) / secondsInitial * 100);
          // Stop countdown if it's finished
          if (!seconds) {
              stop();
              running = false;
          }
      }, 1000);
  }

  // Stop the countdown
  function stop() {
      // Enable slider
      ms.enable();
      // Clear interval
      clearInterval(timer);
      // Reset progress
      root.style.setProperty('--progress', 100);
      // Remove `running` state
      container.classList.remove('container--running');
  }

})();