Vue

HTML

<div id="app">
  <test :ts="ts"></test>
</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

Vue.component('test', {
	template: `
  	<div>{{ last() }}</div>
  `,
 
	props: {
    ts: Number
  },

  data: () => ({
    now: Date.now(),
    gg: 0
  }),
  
  created() {
    setInterval(() => { 
    	this.$forceUpdate();
    }, 3000);
  },
  methods: {
  	last: function() {
      return ~~(-(this.ts - (new Date())) / 1000) + ' секунд';
    }
  }
})

new Vue({
  el: "#app",
  data: {
     ts: +new Date()
  },
  created() {
  	setInterval(() => {
    	console.warn('change');
    	this.ts = +new Date();
    }, 7000);
  },
  methods: {
  	 
  }
})