How to implement Vue timer

<div class="time" v-if="rptType">{{str}}</div>
<div class="receipt" :class="rptType?'jdz':'jiedan'" @click="receipt">Start</div>

data(){
 return {
  rptType: false, //Status h: 0, //Define hours, minutes, seconds, milliseconds and initialize them to 0;
        m:0,
        ms:0,
        s:0,
        time:0,
        str:'',
 }
},
mounted:function(){
 this.$nextTick(function () {//The entire view is rendered})
},
methods:{
 getTask: function(e){
  this.taskType = e;
 },
 //Start receipt: function() {
  this.rptType = !this.rptType;
  if(this.rptType){
   this.time=setInterval(this.timer,50);
  }else{
   this.reset()
  }
 },
 timer: function(){ //define timing function this.ms=this.ms+50; //millisecondsif(this.ms>=1000){
       this.ms=0;
       this.s=this.s+1; //seconds}
     if(this.s>=60){
       this.s=0;
       this.m=this.m+1; //minutes}
     if(this.m>=60){
       this.m=0;
       this.h=this.h+1; //hours}
     this.str =this.toDub(this.h)+":"+this.toDub(this.m)+":"+this.toDub(this.s)+""/*+this.toDubms(this.ms)+"milliseconds"*/;
     // document.getElementById('mytime').innerHTML=h+"hour"+m+"minute"+s+"second"+ms+"millisecond";
 },
 toDub: function(n){ //add 0 operation if(n<10){
       return "0"+n;
     }
     else {
       return ""+n;
     }
 },
 reset: function(){ //Reset clearInterval(this.time);
     this.h=0;
     this.m=0;
     this.ms=0;
     this.s=0;
     this.str="00:00:00";
 },
}