An Optional DateValue for BetterCodable, by duplicating existing types :P

OptionalDateValue.swift

public protocol OptionalDateValueCodableStrategy {
  associatedtype RawValue: Codable

  static func decode(_ value: RawValue?) throws -> Date?
  static func encode(_ date: Date?) -> RawValue?
}

@propertyWrapper
public struct OptionalDateValue<Formatter: OptionalDateValueCodableStrategy>: Codable {
  private let value: Formatter.RawValue?
  public var wrappedValue: Date?

  public init(wrappedValue: Date?) {
    self.wrappedValue = wrappedValue
    self.value = Formatter.encode(wrappedValue)
  }

  public init(from decoder: Decoder) throws {
    self.value = try? Formatter.RawValue(from: decoder)
    self.wrappedValue = try? Formatter.decode(value)
  }

  public func encode(to encoder: Encoder) throws {
    try value.encode(to: encoder)
  }
}

public struct OptionalTimestampStrategy: OptionalDateValueCodableStrategy {
    public static func decode(_ value: TimeInterval?) throws -> Date? {
      return value.map { Date(timeIntervalSince1970: $0) }
    }

    public static func encode(_ date: Date?) -> TimeInterval? {
        return date?.timeIntervalSince1970
    }
}

struct Fixture: Codable {
  @OptionalDateValue<OptionalTimestampStrategy> var timestamp: Date?
}

func testDecodingTimestamp() {
  let jsonData = #"{"timestamp": 851042397.0}"#.data(using: .utf8)!

  let fixture = try! JSONDecoder().decode(Fixture.self, from: jsonData)
  assert(fixture.timestamp == Date(timeIntervalSince1970: 851042397))
  assert(fixture.timestamp?.timeIntervalSince1970 == 851042397)
}

func testDecodingNil() {
  let jsonData = #"{"timestamp": null}"#.data(using: .utf8)!

  let fixture = try! JSONDecoder().decode(Fixture.self, from: jsonData)
  assert(fixture.timestamp == nil)
}

testDecodingTimestamp()
testDecodingNil()