title: ceiling time_point to runtime defined duration
body: <p>I'm ceiling time_point to next full five minutes, which is quite easy:</p>
  
  <pre><code>template &lt;int N&gt;
      using minutes = std::chrono::duration&lt;int, std::ratio_multiply&lt;std::ratio&lt;N&gt;, std::chrono::minutes::period&gt;&gt;;
  
  const auto now = std::chrono::system_clock::now();
  const auto begin = ceil&lt;minutes&lt;5&gt;&gt;(now);
  </code></pre>
  
  <p>I don't know how to set the count of minutes at runtime.</p>