如何使用nstimer显示秒数？

- (void)timer:(NSTimer *)timer {
    NSInteger secondsSinceStart = (NSInteger)[[NSDate date] timeIntervalSinceDate:startDate];

    NSInteger seconds = secondsSinceStart % 60;
    NSInteger minutes = (secondsSinceStart / 60) % 60;
    NSInteger hours = secondsSinceStart / (60 * 60);
    NSString *result = nil;
    if (hours > 0) {
        result = [NSString stringWithFormat:@\"%02d:%02d:%02d\", hours, minutes, seconds];
    }
    else {
        result = [NSString stringWithFormat:@\"%02d:%02d\", minutes, seconds];        
    }
    // set result as label.text
}

self.startDate = [NSDate date];
timer = [NSTimer scheduledTimerWithTimeInterval:0.25 target:self selector:@selector(timer:) userInfo:nil repeats:YES];

self.startDate = nil;
[timer invalidate];
timer = nil;

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

- (void)timerFireMethod:(NSTimer*)theTimer {
    //remove a second from the display
}