Javascript 角度反转定时器和md选择

我想创建一个循环反向定时器，用户应该能够选择原点号。在my md select中，应选择等于10的值。我试图通过使用变量count解决此问题，但它与选择的第一个值time=10关联。。。我认为需要使用计数器，但不知道如何正确操作。或者我需要添加设置为md select或与以下值相关联的selectedTime。怎么做

    import { Component, OnInit } from '@angular/core';
    import { SimpleTimer } from 'ng2-simple-timer';

    @Component({
      selector: 'app-main-map',
      templateUrl: './main-map.component.html',
      styleUrls: ['./main-map.component.css'],

    })
    export class MainMapComponent implements OnInit {
    selectedTime: number = 10;
// counter: number = this.selectedTime;

 up_times = [
    {value: '999', viewValue: 'Не оновлювати'},
    {value: '5', viewValue: '5 sec'},
    {value: '10', viewValue: '10 sec'},
    {value: '15', viewValue: '15 sec'},
    {value: '30', viewValue: '30 sec'},
    {value: '60', viewValue: '1 min'},
    {value: '300', viewValue: '5 min'},
    {value: '600', viewValue: '10 min'},
  ];
    constructor(private st: SimpleTimer) { }

  ngOnInit() {
    this.st.newTimer('1sec',1);
    this.subscribeTimer0();

  }

  subscribeTimer0() {
    if (this.selectedTime == 999) {
      this.st.unsubscribe(this.timer0Id);
      this.timer0Id = undefined;

    } else {

      this.timer0Id = this.st.subscribe('1sec', e => this.timer0callback());

    }
    console.log(this.st.getSubscription());

  }


  timer0callback() {
    this.selectedTime--;
    if(this.selectedTime == 0){
      this.selectedTime = selectedTime;
    }
  }
 // timer0callback() {
  //   this.count--;
  //   if(this.count == 0){
  //     this.count = selectedTime;
  //   }
  // }

我的html：

<md-select [(ngModel)]="selectedTime" >
          <md-option *ngFor="let up_time of up_times" [value]="up_time.value">
            {{ up_time.viewValue }}
          </md-option>
</md-select>

<div style="top: 13px;position: absolute;left: 127px;">{{selectedTime}}</div>
<!--<div style="top: 13px;position: absolute;left: 127px;">{{count}}</div>-->

{{up_time.viewValue}}
{{selectedTime}}