gistfile1.m

    _clockFormat = [[NSDateFormatter alloc] init];
    [_clockFormat setTimeStyle:NSDateFormatterLongStyle];
    [_clockFormat setLocale:[NSLocale systemLocale]];
    
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    _clockTimer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    
    dispatch_source_set_timer(_clockTimer, DISPATCH_TIME_NOW, kClockUpdateInterval * NSEC_PER_SEC, 0);
    dispatch_source_set_event_handler(_clockTimer, ^{
        self.clock.text = [_clockFormat stringFromDate:[NSDate date]];
        NSLog(@"%@",[_clockFormat stringFromDate:[NSDate date]]);
    });
    
    // now that our timer is all set to go, start it
    dispatch_resume(_clockTimer);