CoreDataのwillSave()を使う際はsetPrimitiveValue(:forkey:)を使う

ContentView.swift

import SwiftUI
import CoreData

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    var body: some View {
        NavigationView {
            List {
                ForEach(items) { item in
                    NavigationLink {
                        Text("Item at \(item.timestamp!, formatter: itemFormatter)")
                    } label: {
                        VStack {
                            HStack {
                                Text("Timestamp")
                                Spacer()
                                Text(item.timestamp!, formatter: itemFormatter)
                            }
                            HStack {
                                Text("Created at")
                                Spacer()
                                if let t = item.createdAt {
                                    Text(t, formatter: itemFormatter)
                                }
                            }
                            HStack {
                                Text("Updated at")
                                Spacer()
                                if let t = item.updatedAt {
                                    Text(t, formatter: itemFormatter)
                                }
                            }
                        }
                    }
                }
                .onDelete(perform: deleteItems)
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    EditButton()
                }
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            Text("Select an item")
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()

            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }

    private func deleteItems(offsets: IndexSet) {
        withAnimation {
            offsets.map { items[$0] }.forEach(viewContext.delete)

            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}

private let itemFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    formatter.timeStyle = .medium
    return formatter
}()

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
    }
}

Item+CoreDataClass.swift

import Foundation
import CoreData

@objc(Item)
public class Item: NSManagedObject {

    override public func willSave() {
        super.willSave()
        
        let now = Date()
        if createdAt == nil {
            setPrimitiveValue(now, forKey: "createdAt")
        }
        setPrimitiveValue(now, forKey: "updatedAt")

    }
}

Item+CoreDataProperties.swift

import Foundation
import CoreData


extension Item {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Item> {
        return NSFetchRequest<Item>(entityName: "Item")
    }

    @NSManaged public var timestamp: Date?
    @NSManaged public var createdAt: Date?
    @NSManaged public var updatedAt: Date?

}

extension Item : Identifiable {

}