title: Get the current date and time with milliseconds with only the standard library in C++
body: <p>I'm trying to print a timestamp like this.</p>
  
  <pre><code>2018-05-24T20:16:07.339271
  </code></pre>
  
  <p>I don't want to use Boost or any third party libraries. I want to only use the standard library. I'm using Clang 6, so I should be able to use C++ 17 if necessary.</p>
  
  <p>I started looking at <code>chrono</code> and have something like this.</p>
  
  <pre><code>auto now = std::chrono::high_resolution_clock::now();
  </code></pre>
  
  <p>But, I'm unsure how to get the datetime format that I want from above.</p>