title: Computing average time given set of start and end dates
body: <p>I have a table with a field called <code>date_start</code> and a field called <code>date_end</code> The values of the field are like this</p>
  
  <pre><code>year-month-day hours:minutes:seconds
  </code></pre>
  
  <p>I need to find out:</p>
  
  <ul>
  <li>How much time it to for the person to complete the task.</li>
  <li>The average time it took of all of them.</li>
  </ul>
  
  <p>I'm still a bit new and honestly have no idea where to even start with this. </p>
  
  <p>Thanks for all the help.</p>