- (void)viewDidLoad { 
    
    secondsCountDown = 172800;//倒计时秒数
    countDownTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(countDownAction) userInfo:nil repeats:YES]; //启动倒计时后会每秒钟调用一次方法 countDownAction
    //设置倒计时显示的时间
    NSString *str_hour = [NSString stringWithFormat:@"%02ld",secondsCountDown/3600];//时
    NSString *str_minute = [NSString stringWithFormat:@"%02ld",(secondsCountDown%3600)/60];//分
    NSString *str_second = [NSString stringWithFormat:@"%02ld",secondsCountDown%60];//秒
    NSString *format_time = [NSString stringWithFormat:@"%@:%@:%@",str_hour,str_minute,str_second];
    NSLog(@"time:%@",format_time);

    self.timeLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, bigImageView.frame.size.width, bigImageView.frame.size.height)];
    self.timeLabel.text = [NSString stringWithFormat:@"倒计时   %@",format_time];
    self.timeLabel.textAlignment = NSTextAlignmentCenter;
    self.timeLabel.font = FONT_19;
    self.timeLabel.textColor = ［UIColor  redColor］;
    [self.View addSubview:self.timeLabel];
    }

-(void) countDownAction{
    //倒计时-1
    secondsCountDown--;
    NSString *str_hour = [NSString stringWithFormat:@"%02ld",secondsCountDown/3600];
    NSString *str_minute = [NSString stringWithFormat:@"%02ld",(secondsCountDown%3600)/60];
    NSString *str_second = [NSString stringWithFormat:@"%02ld",secondsCountDown%60];
    NSString *format_time = [NSString stringWithFormat:@"%@:%@:%@",str_hour,str_minute,str_second];
    //修改倒计时标签现实内容
    self.timeLabel.text=[NSString stringWithFormat:@"倒计时   %@",format_time];
    //当倒计时到0时，做需要的操作，比如验证码过期不能提交
    if(secondsCountDown==0){
        [countDownTimer invalidate];
    }
    }

附上github地址:https://github.com/ZSyingyu/ZYYCountDownDemo.git