swift エラーメッセージ

前提・実現したいこと

課題期限通知アプリを作りたいです

発生している問題・エラーメッセージ

Could not cast value of type '__NSDate' (0x104c92080) to 'NSString' (0x10429dc60).```  

viewDidLoadでエラーが起こっているようです  

該当のソースコード

import UIKit  

class FirstViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {  

@IBOutlet var tableView: UITableView!  
var todoarray = [String]()  
var todoarraydate = [String]()  
var refresher: UIRefreshControl?  

override func viewDidLoad() {  
super.viewDidLoad()  
if UserDefaults.standard.array(forKey: "todoarray") == nil {  
todoarray = [String]()  
UserDefaults.standard.set(todoarray, forKey: "todoarray")  
} else {  
todoarray = UserDefaults.standard.array(forKey: "todoarray") as! [String]  
}  

if UserDefaults.standard.array(forKey: "todoarraydate") == nil {  
todoarraydate = [String]()  
UserDefaults.standard.set(todoarraydate, forKey: "todoarraydate")  
} else {  
todoarraydate = UserDefaults.standard.array(forKey: "todoarraydate") as! [String]  
}  

refresher = UIRefreshControl()  

refresher?.attributedTitle = NSAttributedString(string: "Pull to refresh")  

refresher?.addTarget(self, action: #selector(FirstViewController.refresh), for: UIControlEvents.valueChanged)  

self.tableView.addSubview(refresher!)  

refresh()  
// Do any additional setup after loading the view, typically from a nib.  
}  

override func didReceiveMemoryWarning() {  
super.didReceiveMemoryWarning()  
// Dispose of any resources that can be recreated.  
}  

internal func numberOfSections(in tableView: UITableView) -> Int {  
return 1  
}  

func refresh() {  
tableView.reloadData()  
refresher?.endRefreshing()  
}  

internal func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {  
if todoarray.count == todoarraydate.count {  
return todoarray.count  
} else {  
return todoarraydate.count  
}  
}  

internal func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {  
var cell = tableView.dequeueReusableCell(withIdentifier: "cell")  
if cell == nil {  
cell = UITableViewCell(style: .default, reuseIdentifier: "cell")  
}  
cell?.textLabel?.text = todoarray[indexPath.row]  
cell?.detailTextLabel?.text = String(describing: todoarraydate[indexPath.row])  
return cell!  
}  

internal func tableView(_ tableView: UITableView, canEditRowAt indexPath: IndexPath) -> Bool {  
// Return false if you do not want the specified item to be editable.  
return true  
}  

@IBAction func new(_ sender: AnyObject) {  
self.performSegue(withIdentifier: "New To Do", sender: self)  
}  

func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCellEditingStyle, forRowAt indexPath: IndexPath) {  
if editingStyle == .delete {  
todoarray.remove(at: indexPath.row)  
todoarraydate.remove(at: indexPath.row)  
tableView.deleteRows(at: [indexPath], with: .fade)  
UserDefaults.standard.set(todoarray, forKey: "todoarray")  
UserDefaults.standard.set(todoarraydate, forKey: "todoarraydate")  
}  
}  

//画面遷移  
func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {  

performSegue(withIdentifier: "toSubViewController", sender: nil)  
}  

}  

最後に

分からないことがあれば質問してください
よろしくお願いします