title: How to save previous timestamp to Firebase FireStore
body: <p>I want to migrate the older data from different system to firestore.</p>
  
  <p>below command saves current timestamp</p>
  
  <pre><code>firebase.firestore.FieldValue.serverTimestamp()
  </code></pre>
  
  <p>I want to store older values, Is there any way available?</p>