title: Accuracy of Date.now() in browsers
body: <p>I've seen plenty of questions about how to get the date/time in JavaScript, and the answers always look something like</p>
  
  <blockquote>
    <p><code>Date.now()</code> returns the UTC timestamp in milliseconds</p>
  </blockquote>
  
  <p>But how reliable is that value really? Is it just based on the system clock of whatever computer is running the browser? If so, it seems like it could vary arbitrarily far from true UTC time, but how much variation should be expected?. I know caring about accurate milliseconds is a lost cause, but what about on the order of seconds or minutes?</p>