ERROR: Use of undeclared type 'Date'?

protocol FullNameable {
    var fullName: String { get }
}

struct User: FullNameable {
    var fullName: String
}

let user = User(fullName: "John Smith")

struct Friend: FullNameable {
    let firstName: String
    let middleName: String
    let lastName: String

    var fullName: String {
        return "\(firstName) \(middleName) \(lastName)"
    }
}

let friend = Friend(firstName: "Taylor", middleName: "Alison", lastName: "Swift")
friend.fullName

enum EmployeeType {
    case manager
    case traditional
}

class Employee {
    let name: String
    let address: String
    let startDate: Date
    let type: EmployeeType

    init(name: String, address: String, startDate: Date, type: EmployeeType) {
        self.name = name
        self.address = address
        self.startDate = startDate
        self.type = type
    }
}