Swift：当添加到coredata时，NSDate返回nil

Question

当我尝试将NSDate添加到newWorkout时;代码行：

newWorkout.setValue(currentTime, forKey: "workoutDate")

即使我在执行代码行之前打印currentTime，它也返回nil，返回正确的NSDate。保存中的核心数据是名为Workout的实体，其中两个属性为Int16类型的workoutTime和类型为Date的workoutDate。

   let appDel: AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
    let context: NSManagedObjectContext = appDel.managedObjectContext


    let newWorkout = NSEntityDescription.insertNewObjectForEntityForName("Workout", inManagedObjectContext: context) as! Workout

    let totalTimeConverted = Int16(totalTime)

    let currentTime = NSDate()
    print(currentTime)

    // Add parameters
    newWorkout.setValue(NSInteger(totalTimeConverted), forKey: "workoutTime")
    newWorkout.setValue(currentTime, forKey: "workoutDate")


    do {
        try context.save()
    } catch {
        print("Error saving data")
    }


    do {
        let request = NSFetchRequest(entityName: "Workout")
        let results = try context.executeFetchRequest(request)
        if results.count > 0 {

        for item in results as! [NSManagedObject]{

            let time = item.valueForKey("workoutTime")
            print(time)
            let workoutDate = item.valueForKey("workoutDate")
            print(workoutDate)
            print(time!, workoutDate!)

        }
        }
    } catch{
        print("Error loading data")
    }