用nstimer實現倒計時功能，廢話不多說，直接上代碼，詳細解釋請參照注釋

// 
[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFireMethod:) userInfo:nil repeats:YES]; 
 
// 
- (void)timerFireMethod:(NSTimer *)theTimer 
{ 
    BOOL timeStart = YES; 
    NSCalendar *cal = [NSCalendar currentCalendar];//定義一個NSCalendar對象 
    NSDateComponents *endTime = [[NSDateComponents alloc] init];    //初始化目標時間... 
    NSDate *today = [NSDate date];    //得到當前時間 
     
    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) {//從NSDate中取出年月日，時分秒，但是只能取一次 
        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]; //把目標時間裝載入date 
    //用來得到具體的時差，是為了統一成北京時間 
    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(@"===%@天 %@:%@:%@",t,h,fen,miao); 
    [_longtime setText:[NSString stringWithFormat:@"%@天 %@:%@:%@",t,h,fen,miao]]; 
    if([d second] > 0) { 
        //計時尚未結束，do_something 
//        [_longtime setText:[NSString stringWithFormat:@"%@:%@:%@",d,fen,miao]]; 
    } else if([d second] == 0) { 
        //計時結束 do_something 
         
    } else{ 
//計時器失效
        [theTimer invalidate]; 
    } 
     
}