ios - [NSCFString timeIntervalSinceReferenceDate] : unrecognized selector sent to instance

Question

我想比较两个日期之间的差异，但是使用下面的代码我得到了错误“[NSCFString timeIntervalSinceReferenceDate]:无法识别的选择器发送到实例。”代码有什么问题？

NSDate *dateAdded=[eventDictionary objectForKey:@"dateAdded"];
NSDate *validUntilDate=[eventDictionary objectForKey:@"validUntilDate"];
NSDateComponents *sometimeAgo = [[NSCalendar currentCalendar] components:(NSSecondCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit) fromDate:dateAdded  toDate:validUntilDate  options:0]; 

Answer 1

看起来 dateAdded 和/或 validUntilDate 实际上是字符串而不是日期。也许它们是表示日期的字符串，但毕竟是字符串。