title: Doctrine 2. How to calculate difference between two datetime fields in querybuilder
body: <p>I have to following tables:</p>
  
  <pre><code>Registration
      id int
      created datetime
  
  Activity
      id int
      startdate datetime
  </code></pre>
  
  <p>Now I want to add this condition to my SQL query:</p>
  
  <pre><code>(Registration.created + 7 days) &lt;= Activity.startdate
  </code></pre>
  
  <p>How can I do this in Doctrine 2? I prefer a database independent solution.</p>