objective-c - objcで時間を比較する方法

Question

カウントダウンして、Day、Hour、M、Sを表示したいのですが、1秒が1回呼び出されます。

- (void)changetime {     

    NSCalendar *cal = [NSCalendar currentCalendar];//定义一个NSCalendar对象                 
    NSDate *today = [NSDate date];//得到当前时间

    unsigned int unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
    NSDateComponents *weekdayComponents =[cal components:(unitFlags) fromDate:self.overDate]; 
    NSDate *todate = [cal dateFromComponents:weekdayComponents]; 
    NSDateComponents *d = [cal components:unitFlags fromDate:today toDate:todate options:0];

    NSInteger days=[d year]*365+[d month]*31+[d day];
    if ([self.overDate earlierDate:[NSDate date]]==self.overDate) {
        [myTimer invalidate];
        myTimer=nil;
        self.labdate.text = [NSString stringWithFormat:@"%d天%d时%d分%d秒",days, [d hour], [d minute], [d second]];
        return;
    }    
    self.labdate.text = [NSString stringWithFormat:@"%d天%d时%d分%d秒",days, [d hour], [d minute], [d second]];     
}

myTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(changetime) userInfo:nil repeats:YES];

self.overDate=[wedding date];//this is a future time