两个日期之间的时间间隔返回nil

我正在计算两个NSDate之间的时差，它始终返回0.0000。我正在使用相同的逻辑来计算时间戳，并且格式正确。.当我尝试分别记录日期时，它会正确打印，当我打印差异，打印0.0000 ...这是计算日期的代码..

-(NSDate *) toLocalTime
{
NSTimeZone *tz = [NSTimeZone defaultTimeZone];
NSInteger seconds = [tz secondsFromGMTForDate: [NSDate date]];
return [NSDate dateWithTimeInterval: seconds sinceDate: [NSDate date]];
}

这是我发现不同之处的代码。

NSLog(@"the time when the location was updated is %@",timeWhenTheLocationWasUpdated);
NSLog(@"the time when the server was contacted last time was %@",timeWhenLastLocationPosted);

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"dd-MM-yyyy HH:mm:ss ZZZ"];
NSDate *timeofUpdatingLocation = [[NSDate alloc] init];
NSDate *timeofPostingToTheServer = [[NSDate alloc]init];
timeofUpdatingLocation = [dateFormatter dateFromString:timeWhenTheLocationWasUpdated];
timeofPostingToTheServer = [dateFormatter dateFromString:timeWhenLastLocationPosted];
NSTimeInterval difference = [timeofPostingToTheServer timeIntervalSinceDate:timeofUpdatingLocation];
NSLog(@"the difference between posting the location to the server and updating the location is %f",difference);

代码开头的两个NSlog均以日期格式化程序中提到的格式正确打印日期。