title: How to save a timestamp in firestore
body: <p>I save the Map showen below to a firestore, but I want to save also a timestamp which should have the time and date a user picked with a date and time picker. The date is already in 'dd.MM.yyyy' formatted and the time is something like '6:58 PM'. The date is the variable <code>date</code> and the time is the variable <code>time</code>. How can I save a timestamp with the value <code>time</code> and <code>date</code>?</p>
  <pre><code>Map&lt;String, dynamic&gt; data = {
        'name': userName,
        'userUID': userUIDglobal,
        'time: '//here I want to safe the timestamp
      };
      Firestore.instance
          .collection('seller')
          .document(documentID)
          .collection('test')
          .add(data);
    }
  </code></pre>