Objective-C Xcode - if 语句“预期表达式”

问：

我必须检查 2 次之间的间隔是否> 0，然后设置标签的文本，但是当我使用 if 语句时，我得到了“预期表达式”。

NSDate * now = [NSDate date];
NSDateFormatter *outputFormatter = [[NSDateFormatter alloc] init];
[outputFormatter setDateFormat:@"HH:mm:ss"];
NSString *newDateString = [outputFormatter stringFromDate:now];

NSDateFormatter *df = [[NSDateFormatter alloc] init];
[df setDateFormat:@"HH:mm:ss"];
NSDate *date1 = [df dateFromString:newDateString];
NSDate *date2 = [df dateFromString:@"15:00:00"];
NSTimeInterval interval = [date2 timeIntervalSinceDate:date1];
int hours = (int)interval / 3600; // integer division to get the hours part
int minutes = (interval - (hours*3600)) / 60; // interval minus hours part (in seconds) divided by 60 yields minutes
int seconds = interval - ((hours*3600)+(minutes*60)) ;
NSString *timeDiff = [NSString stringWithFormat:@"%02d | %02d | %02d", hours, minutes, seconds];

if (interval < 0)
{
self.before3.text = timeDiff;
}

错误在if (interval < 0)

有人可以帮我吗？

问候。