Vue

HTML

<div id="app">
   {{ last }}
</div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

del {
  color: rgba(0, 0, 0, 0.3);
}

Vue

new Vue({
  el: "#app",
  data: {
    ts: new Date(),
    last: '0 секунд'
  },
  created() {
    setInterval(() => {
    	this.last = ~~(-(this.ts - (new Date())) / 1000) + ' секунд';
    }, 1000);
  } 
})