@propertyWrapper
struct Expirable<Value: ExpressibleByNilLiteral> {

  let duration: TimeInterval
  private var expirationDate = Date()
  private var innerValue: Value = nil

  private var hasExpired: Bool {
    expirationDate < Date()
  }

  init(duration: TimeInterval) {
    self.duration = duration
  }

  var value: Value {
    get {
      hasExpired ? nil : innerValue
    }
    set {
      expirationDate = Date().addingTimeInterval(duration)
      innerValue = newValue
    }
  }

}

struct Tokens {
  @Expirable(duration: 3) static var authentication: String?
}

Tokens.authentication = "abc"

sleep(2)
Tokens.authentication // "abc"
sleep(2)
Tokens.authentication // nil