title: What is the equivalent data type of cloud Firestore server timestamp in swift language?
body: <p>I've got a data model including a timestamp field <code>createdAt</code>.</p>
  
  <pre><code>struct Box {
      var title: String
      var createdAt: Timestamp
  
      var dictionary: [String : Any] {
          return [
              "title": title,
              "createdAt": createdAt
          ]
      }
  
  // ...
  </code></pre>
  
  <p>However, I am unable to assign Firestore server timestamp. What would be the equivalent data type of cloud Firestore server timestamp in swift? Am I missing a point here?</p>
  
  <pre><code>let box = Box(title: title, createdAt: FieldValue.serverTimestamp());
  </code></pre>