将NSDate转换为NSString以保存在NSMutableArray中

I'm very new to iOS programming (as you can probably tell from my code) and I'm having difficulties. My app is a questionnaire with many ViewControllers, each ViewController has a question. The answers to these questions get stored in a NSMutableArray. I then present the answers on the last view, a UITableView. Everything works fine apart from displaying the date in the tableview. I get this error message: I'm very new to iOS programming (as you can pro