app/src/App.vue

<template>
  <div id="app">
    <Settings v-if="time == 0" v-on:set-time="setTime"/>
    <Timer v-if="time != 0" v-bind:time="time"/>
  </div>
</template>

<script>
import Settings from './components/Settings.vue'
import Timer from './components/Timer.vue'

export default {
  name: 'App',
  components: {
    Timer,
    Settings
  },
  data: function() {
    return {
      time: 0
    }
  },
  methods: {
    setTime(number) {
      this.time = number;
    }
  }

}
</script>

<style>
html, body {
  height: 100%;
  margin: 0;
}
#app {
  font-family: "HelveticaNeue", "Helvetica Neue", Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  height: 100%;
}
</style>