title: Firebase Firestore update time field security issue
body: <p>I am developing a simple chat website using Firebase Firestore. And it obvious to store the message time . </p>
  
  <p>Now the thing is that document is added from client side. So malcius user can add document with fake time. Is there any way avoid the scenario.</p>
  
  <p>I have tried using cloud functions but it's taking too long lo send message..</p>