client_id_js.js

<script>
  (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
  (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
  m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
  })(window,document,'script','https://www.google-analytics.com/analytics.js','ga');

  ga('create', 'UA-XXXXXX-XX', 'auto');
  
  // get client id
  ga(function(tracker) {
    var clientId = tracker.get('clientId');
    ga('set', 'dimension1', clientId);
  });
  // get timestamp
  var timeStamp = Math.floor( (new Date).getTime()/1000 );
  ga('set', 'dimension2', timeStamp);
  
  ga('send', 'pageview');
</script>