title: How to save Timestamp value in Cloud Firestore and perform query on it?
body: <p>I've followed this <a href="https://stackoverflow.com/questions/54217229/how-to-save-timestamp-in-firestore">Stackoverflow post</a> and there is clearly said how to store the timestamp, if we user the <strong>ServerTimestamp</strong> annotation. However, I want to ask about other alternatives, which can later also be used to perform queries on them.</p>
  <p>With respect to the different time libraries (before/after Java 8) we can get the timestamp as a <code>long</code> variables, or as <code>ZonedDateTime</code> object. My question is:</p>
  <p>Can we save <code>ZonedDateTime</code> object or a <code>long</code> variable as a String and then perform queries on it, or we must use in any case <strong>ServerTimestamp</strong> annotation?</p>