title: How to insert server timestamp in Firestore document using REST?
body: <p>I want to insert a document using a REST call to <a href="https://firebase.google.com/docs/firestore/reference/rest/v1beta1/projects.databases.documents/createDocument" rel="noreferrer">Firestore createDocument</a> method. One of the fields is a timestamp field that should be set on the server. With Android SDK it's as simple as annotating a <code>Date</code> field with <code>@ServerTimestamp</code> and keeping it null — now how do I do it in REST?</p>
  
  <pre><code>{
    "fields": {
      "timezoneId": {
        "stringValue": "Europe\/London"
      },
      "city": {
        "stringValue": "London"
      },
      "timestamp": {
        "timestampValue": "???"
      }
    }
  }
  </code></pre>
  
  <p>I tried using <code>null</code>, <code>0</code>, empty string, <code>timestamp</code> — everything fails with an error requiring the standard RFC3339 format (e.g. <code>2018-01-31T13:50:30.325631Z</code>). Is there any placeholder value I can use, or any way to obtain that timestamp?</p>