title: Using Firebase server timestamp to create a synchronized countdown timer on Android
body: <p>I want to display a count down timer unified for all users.
  I choose a way to do it but i'm not sure if it is the best option.</p>
  
  <p>server side:</p>
  
  <ol>
  <li>node server calls to Firebase with a server timestamp request.</li>
  <li>server added X millis to timestamp</li>
  <li>server updates Firebase db with the the new future timestamp</li>
  </ol>
  
  <p>user side:</p>
  
  <ol>
  <li>receives a a future timestamp from db (created by the server)</li>
  <li>send a request to Firebase db with to receive the current server timestamp</li>
  <li>calculates delta between server timestamp and current timestamp (local)</li>
  <li>add the calculated delta to future timestamp received in stage 1</li>
  <li>displays the corrected  count down time left.</li>
  </ol>
  
  <p>The main issue i see with this method is that the user have unlimited read/write access to "timestamp" value. Can this be used against me to make fake calls to db thus increasing my Firebase usage (costing me money)</p>
  
  <p>Is there a better way syncing all devices into a unified time in the future?</p>