如何实现一个倒计时器？

TextTimer({ controller: this.textTimerController, isCountDown: this.isCountDown, count: 30000 })
  .format(this.format)
  .fontColor(Color.Black)
  .fontSize(50)
  .onTimer((utc: number, elapsedTime: number) => {
    console.log(`textTimer notCountDown utc is：${utc}, elapsedTime: ${elapsedTime}`);
  })
Button("Start").onClick(() => {
  this.textTimerController.start();
})