问NSDate返回负NSTimeInterval

EN

- (void)pointsStartedDisplaying {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            self.timeStartedDisplayingPoint = [NSDate date];
            self.stopButton.enabled = YES;

            [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(updateElapsedTime:) userInfo:nil repeats:YES];
        });
    });
}

- (void)updateElapsedTime:(NSTimer *)timer {
    NSTimeInterval elapsedTimeDisplayingPoints = [self.timeStartedDisplayingPoint timeIntervalSinceDate:[NSDate date]];
    self.elapsedTimeValueLabel.text = [NSString stringWithFormat:@"%f seconds", elapsedTimeDisplayingPoints];
}