IOS developed code sharing with nstimer to achieve the countdown function

With nstimer to achieve the countdown function, nonsense, directly on the code, please refer to the notes for detailed explanation

// 
[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFireMethod:) userInfo:nil repeats:YES]; 
 
// 
- (void)timerFireMethod:(NSTimer *)theTimer 
{ 
    BOOL timeStart = YES; 
    NSCalendar *cal = [NSCalendar currentCalendar];// define 1 a NSCalendar object  
    NSDateComponents *endTime = [[NSDateComponents alloc] init];    // Initialize the target time ... 
    NSDate *today = [NSDate date];    // Get the current time  
     
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; 
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"]; 
    NSDate *dateString = [dateFormatter dateFromString:todate]; 
    NSString *overdate = [dateFormatter stringFromDate:dateString]; 
//    NSLog(@"overdate=%@",overdate); 
    static int year; 
    static int month; 
    static int day; 
    static int hour; 
    static int minute; 
    static int second; 
    if(timeStart) {// from NSDate Middle take out the day, month, hour, minute, second, but can only take 1 time  
        year = [[overdate substringWithRange:NSMakeRange(0, 4)] intValue]; 
        month = [[overdate substringWithRange:NSMakeRange(5, 2)] intValue]; 
        day = [[overdate substringWithRange:NSMakeRange(8, 2)] intValue]; 
        hour = [[overdate substringWithRange:NSMakeRange(11, 2)] intValue]; 
        minute = [[overdate substringWithRange:NSMakeRange(14, 2)] intValue]; 
        second = [[overdate substringWithRange:NSMakeRange(17, 2)] intValue]; 
        timeStart= NO; 
    } 
     
    [endTime setYear:year]; 
    [endTime setMonth:month]; 
    [endTime setDay:day]; 
    [endTime setHour:hour]; 
    [endTime setMinute:minute]; 
    [endTime setSecond:second]; 
    NSDate *overTime = [cal dateFromComponents:endTime]; // Load in the target time date 
    // To get the exact time difference, for unification 1 Formation time  
    unsigned int unitFlags = NSYearCalendarUnit| NSMonthCalendarUnit| NSDayCalendarUnit| NSHourCalendarUnit| NSMinuteCalendarUnit| NSSecondCalendarUnit; 
    NSDateComponents *d = [cal components:unitFlags fromDate:today toDate:overTime options:0]; 
    NSString *t = [NSString stringWithFormat:@"%d", [d day]]; 
    NSString *h = [NSString stringWithFormat:@"%d", [d hour]]; 
    NSString *fen = [NSString stringWithFormat:@"%d", [d minute]]; 
    if([d minute] < 10) { 
        fen = [NSString stringWithFormat:@"0%d",[d minute]]; 
    } 
    NSString *miao = [NSString stringWithFormat:@"%d", [d second]]; 
    if([d second] < 10) { 
        miao = [NSString stringWithFormat:@"0%d",[d second]]; 
    } 
//    NSLog(@"===%@ day  %@:%@:%@",t,h,fen,miao); 
    [_longtime setText:[NSString stringWithFormat:@"%@ day  %@:%@:%@",t,h,fen,miao]]; 
    if([d second] > 0) { 
        // The clock is not up yet, do_something 
//        [_longtime setText:[NSString stringWithFormat:@"%@:%@:%@",d,fen,miao]]; 
    } else if([d second] == 0) { 
        // End of the timing  do_something 
         
    } else{ 
// Timer failure 
        [theTimer invalidate]; 
    } 
     
}