用nstimer实现倒计时功能，废话不多说，直接上代码，详细解释请参照注释

// [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFireMethod:) userInfo:nil repeats:YES];  // - (void)timerFireMethod:(NSTimer *)theTimer {     BOOL timeStart = YES;     NSCalendar *cal = [NSCalendar currentCalendar];//定义一个NSCalendar对象     NSDateComponents *endTime = [[NSDateComponents alloc] init];    //初始化目标时间...     NSDate *today = [NSDate date];    //得到当前时间          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) {//从NSDate中取出年月日，时分秒，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]; //把目标时间装载入date     //用来得到具体的时差，是为了统一成北京时间     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) {         //计时尚未结束，do_something //        [_longtime setText:[NSString stringWithFormat:@"%@:%@:%@",d,fen,miao]];     } else if([d second] == 0) {         //计时结束 do_something              } else{ //计时器失效        [theTimer invalidate];     }      }