Как остановить таймер, когда он дойдет до 00:00:00 и начать считать 'наверх'?

<div class="green--text" v-if="operation.idle_el === 1" v-html="getDiff()"></div>

data () {
return {
  moment: moment.locale('ru'),
  diff: null,
  startTime: null,
  endTime: moment('20.08.19 17:50:00', 'DD.MM.YY HH:mm:ss').format('LTS')
}},
methods: {
getDiff () {
  this.diff = moment.utc(moment.duration(this.endTime) - moment.duration(this.startTime)).format('LTS')
  console.log(this.diff)
  // return '<div>' + this.diff + '</div>'
  return this.diff
},
updateCurrentTime () {
  this.startTime = moment().format('LTS')
}
},
created () {
this.startTime = moment().format('LTS')
setInterval(() => this.updateCurrentTime(), 1 * 1000)
}