[NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance

I would like to compare difference between two dates, but with the code below I got the error "[NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance." What's wrong with the code?

NSDate *dateAdded=[eventDictionary objectForKey:@"dateAdded"];
NSDate *validUntilDate=[eventDictionary objectForKey:@"validUntilDate"];
NSDateComponents *sometimeAg开发者_高级运维o = [[NSCalendar currentCalendar] components:(NSSecondCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit) fromDate:dateAdded  toDate:validUntilDate  options:0]; 

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It looks like dateAdded and/or validUntilDate are actually strings and not dates. Maybe they are strings representing dates, but strings after all.

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You need to use an NSDateFormatter to convert your date strings to actual NSDates.