示例代碼簡單易懂：

#import 

@interface ViewController : UIViewController

@property (weak, nonatomic) IBOutlet UILabel *dayLabel;
@property (weak, nonatomic) IBOutlet UILabel *hourLabel;
@property (weak, nonatomic) IBOutlet UILabel *minuteLabel;
@property (weak, nonatomic) IBOutlet UILabel *secondLabel;

@end

#import "ViewController.h"

@interface ViewController ()
{
     dispatch_source_t _timer;
}
@end

@implementation ViewController
/**
 *  獲取當天的年月日的字符串
 *  這裡測試用
 *  @return 格式為年-月-日
 */
-(NSString *)getyyyymmdd{
    NSDate *now = [NSDate date];
    NSDateFormatter *formatDay = [[NSDateFormatter alloc] init];
    formatDay.dateFormat = @"yyyy-MM-dd";
    NSString *dayStr = [formatDay stringFromDate:now];
    
    return dayStr;
    
}
- (void)viewDidLoad {
    [super viewDidLoad];
    NSDateFormatter *dateFormatter=[[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd"];
    
    NSDate *endDate = [dateFormatter dateFromString:[self getyyyymmdd]];
    NSDate *endDate_tomorrow = [[NSDate alloc] initWithTimeIntervalSinceReferenceDate:([endDate timeIntervalSinceReferenceDate] + 24*3600)];
    NSDate *startDate = [NSDate date];
    NSTimeInterval timeInterval =[endDate_tomorrow timeIntervalSinceDate:startDate];
    
    if (_timer==nil) {
        __block int timeout = timeInterval; //倒計時時間
        
        if (timeout!=0) {
            dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
            _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
            dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒執行
            dispatch_source_set_event_handler(_timer, ^{
                if(timeout<=0){ //倒計時結束，關閉
                    dispatch_source_cancel(_timer);
                    _timer = nil;
                    dispatch_async(dispatch_get_main_queue(), ^{
                        self.dayLabel.text = @"";
                        self.hourLabel.text = @"00";
                        self.minuteLabel.text = @"00";
                        self.secondLabel.text = @"00";
                    });
                }else{
                    int days = (int)(timeout/(3600*24));
                    if (days==0) {
                        self.dayLabel.text = @"";
                    }
                    int hours = (int)((timeout-days*24*3600)/3600);
                    int minute = (int)(timeout-days*24*3600-hours*3600)/60;
                    int second = timeout-days*24*3600-hours*3600-minute*60;
                    dispatch_async(dispatch_get_main_queue(), ^{
                        if (days==0) {
                            self.dayLabel.text = @"0天";
                        }else{
                            self.dayLabel.text = [NSString stringWithFormat:@"%d天",days];
                        }
                        if (hours<10) {
                            self.hourLabel.text = [NSString stringWithFormat:@"0%d",hours];
                        }else{
                            self.hourLabel.text = [NSString stringWithFormat:@"%d",hours];
                        }
                        if (minute<10) {
                            self.minuteLabel.text = [NSString stringWithFormat:@"0%d",minute];
                        }else{
                            self.minuteLabel.text = [NSString stringWithFormat:@"%d",minute];
                        }
                        if (second<10) {
                            self.secondLabel.text = [NSString stringWithFormat:@"0%d",second];
                        }else{
                            self.secondLabel.text = [NSString stringWithFormat:@"%d",second];
                        }
                        
                    });
                    timeout--;
                }
            });
            dispatch_resume(_timer);
        }
    }


}
效果：