FieldValue.serverTimestamp()で入ったデータを変換

nilなので

        let tmpTimeStamp: Timestamp? = data["date"] as? Timestamp
        if let timeStamp = tmpTimeStamp {
            date = timeStamp.dateValue()
        } else if let tmpDate = message.get("date", serverTimestampBehavior: .estimate) as? Timestamp {
            // FieldValue.serverTimestamp()で入ったデータを変換
            // https://qiita.com/oYuu/items/fa7479a37f49cf7fc294
            date = tmpDate.dateValue()
        }