title: Google Analytics Tracking Code: Confirming Parallel-Run of Old ga.js & New gtag.js
body: <p>All the while, I've been using the old classic <code>ga.js</code> (e.g. <code>_gaq.push()</code>) to generate monthly traffic report for my clients. Now, I decided to migrate over to the latest <code>gtag.js</code> in order to replace my <code>custom variables</code> with the new <code>custom dimensions</code> (or probably <code>custom metrics</code> as well). But one problem here:</p>
  
  <ul>
  <li><strong>How do I handle the Month Transition for the accuracy of report data?</strong></li>
  </ul>
  
  <p>When I think of <strong>parallel run of old and new tracking codes</strong> (at least 1 week before the coming month), I found this on <a href="https://support.google.com/analytics/answer/1032400?hl=en" rel="nofollow noreferrer">Google Analytics Help</a>:</p>
  
  <blockquote>
    <p>You can, for example, install multiple instances of the Universal
    Analytics tag (analytics.js) on your web pages but <strong>only one instance</strong>
    <strong>of the Classic Analytics code (ga.js)</strong>. Multiple instances of ga.js
    might result in inaccurate data collection, processing, or reporting.
    You can, however, install one or more instances of analytics.js on web
    pages that <strong>also have a single instance of ga.js</strong>.</p>
  </blockquote>
  
  <p>Therefore, I would like confirm this with some experts here: <strong>Is it SAFE to go ahead with one instance of ga.js and one instance of gtag.js on the same page?</strong> Note that if I don't implement old and new GA codes at the same time, let's say 1 week before the coming month, my <strong>custom variables</strong> will lose 1 week data; meanwhile, if I use the new report, my <strong>custom dimensions</strong> will lose 3 weeks data. Also, I just can't simply implement the new code on <strong>May 1, 2019 00:00 AM</strong> -- What if my new implementation doesn't work? I need several days to monitor it as well.</p>
  
  <p>So this is important to me. I wonder if anyone of you here has experienced this and can assure me that this is <strong>safe to go ahead</strong>. Thank you so much!</p>