title: Converting epoch to UTC date format using date.h
body: <p>A very simple question. I have been using date.h to get epoch in milliseconds and nanoseconds from a full datestamp.</p>
  <pre><code>     istringstream temp_ss1{timestamp1};
       sys_time&lt;nanoseconds&gt; tp1;
       temp_ss1 &gt;&gt; parse(&quot;%Y/%m/%d,%T&quot;, tp1);
       std::cout &lt;&lt; tp1.time_since_epoch().count() &lt;&lt; &quot;ns\n&quot;;
  </code></pre>
  <p>What is the method to go from a millisecond epoch to a full datestamp with milliseconds?</p>
  <p>Thank you very much for your help !</p>