extension Notification.Name {
    static let testNotification = Notification.Name("test")
}

class Foo {
    // 1. Changing type to Int, String, etc. prevents crash
    var whyDoIcrashOnlyWithDate: Date?

    init() {
        NotificationCenter.default.addObserver(
          self, 
          selector: #selector(handleNotification), 
          name: .testNotification, 
          object: nil
        )
    }
}
// 2. Having handling in extension is important, doing it in class prevents crash
extension Foo {
    @objc func handleNotification() {}
}

// execution
NotificationCenter.default.post(name: .testNotification, object: nil)

NSForwarding: warning: object 0x60000113eb00 of class 'RuntimeCrash.Foo' does not implement methodSignatureForSelector: -- trouble ahead
Unrecognized selector -[RuntimeCrash.Foo handleNotification]