highly customised universal analytics snippet with cross domain, cross device and autotracking support

big_snippet.js

<!-- Universal Analytics start -->
<script async src='path/to/autotrack.js'></script>
<script>
var userID = "123";	// (set the user ID here via php etc.)

function getClientID() {
  return ga(function(tracker) { 
    var clientId = tracker.get('clientId');
  });
}

function genSessionID() {
      return new Date().getTime() + '.' + Math.random().toString(36).substring(5);
}

function genTimeStamp() {
    // Get local time as ISO string with offset at the end
    var now = new Date();
    var tzo = -now.getTimezoneOffset();
    var dif = tzo >= 0 ? '+' : '-';
    var pad = function(num) {
        var norm = Math.abs(Math.floor(num));
        return (norm < 10 ? '0' : '') + norm;
    };
    return now.getFullYear() 
        + '-' + pad(now.getMonth()+1)
        + '-' + pad(now.getDate())
        + 'T' + pad(now.getHours())
        + ':' + pad(now.getMinutes()) 
        + ':' + pad(now.getSeconds())
        + '.' + pad(now.getMilliseconds())
        + dif + pad(tzo / 60) 
        + ':' + pad(tzo % 60);
}
  (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','//www.google-analytics.com/analytics.js','ga');
  ga('create', 'UA-XXXXXX-YY', 'auto', {		  //create tracker + configuration objects
		'allowLinker': true , 'userId': userID	  // cross domain && cross device user_id trackin
   });	
	//plugins
	
	ga('require', 'displayfeatures');			      // doubleclick cookie AKA display feature
	ga('require', 'linker');			              // cross domain plugin 
	ga('require', 'linkid');                    // enhanced link attribution plugin
	ga('require', 'autotrack');                 // autotraking plugins
	ga('linker:autoLink', ['main.domain.com', 'otherdomain.co.il', 'domain.com']);
	//custom dimension to complete schema
  ga('set', { 'dimension0': userID});  		          //set user ID  (user scope var)
  ga('set', { 'dimension1': getClientID() });  		  //set clientID (session scope var)
  ga('set', { 'dimension2': genSessionID() });  		//set sessionID (session scope var)
  ga('set', { 'dimension3': genTimeStamp() });  		//set time stamp (hit scope var)
  
  //ga('require', 'ecommerce');                     // ecommerce or
  //ga('require', 'ec');                            // enhanced ecommerce
  
  //set content groups
  
  if (document.location.pathname.indexOf('user/' + userID) > -1) {  //remove userID from page to reduce cardinality
    var page = document.location.pathname.replace('user/' + userID, 'user');
    ga('send', 'pageview', page);
  }else{
    ga('send', 'pageview');				//send a hit
  }
  
  //send impressions + promotions 
</script>
<!-- Universal Analytics end -->

based in parts on:

• http://www.simoahava.com/analytics/improve-data-collection-with-four-custom-dimensions/#2
• http://stackoverflow.com/questions/17415579/how-to-iso-8601-format-a-date-with-timezone-offset-in-javascript/17415677#17415677