title: Flutter: query Firestore document by timestamp
body: <p>I'm trying to query Firestore documents by a Timestamp field (e.g. between a start and end date).
  Timestamp field returns something like <code>Timestamp(seconds=1599170400, nanoseconds=0)</code>.</p>
  <p>I have a Unix timestamp (in seconds) in my app which I like to compare this to (using <em>isLessThan:</em>), but the value in Firestore apparently is inside this Timestamp object.</p>
  <p>How would I go about comparing these two timestamps?</p>
  <p>An alternative would be to query all documents and compare afterwards, but I would like to filter the documents in the query itself.</p>