如何清除NSTimer内存?

我正在使用 NSTimer 录制时在标签上显示秒数。 我在点击录制按钮时启动计时器,在点击停止按钮时停止计时器。 当我点击播放按钮时,计时器不会重新启动。

-(void)startTimer
{
 playTimer = [NSTimer scheduledTimerWithTimeInterval:1
                                             target:self
                                           selector:@selector(timerController)
                                           userInfo:nil
                                            repeats:YES];
}
- (NSString*)getTimeStr : (int) secondsElapsed
{
seconds = secondsElapsed % 60;
int minutes = secondsElapsed / 60;
int hours = secondsElapsed/3600;
return [NSString stringWithFormat:@"%02d:%02d:%02d", hours, minutes, seconds];
}

- (void)timerController{
seconds++;
[[self timeLabel] setText:[self getTimeStr:(seconds)]];
}

- (IBAction)recordTapped:(id)sender {
[self startTimer];
}

- (IBAction)stopTapped:(id)sender {
 [playTimer invalidate];
  playTimer=nil;
 _timeLabel.text=@"00:00:00";
}

- (IBAction)playTapped:(id)sender {
[self startTimer];
}