title: Group by week returning strange intervals
body: <p>For some odd reason, group by week is returning odd date intervals with a datetime field. </p>
  
  <p>"Completed" is a datetime field, and using this query:</p>
  
  <pre><code>SELECT 
      Completed, 
      COUNT( DISTINCT Table1.ID ) AS ActivityCount 
  FROM Table1 
  JOIN Table1Items 
  ON Table1.ID = Table1Items.ID 
  JOIN database_database.Table2 
  ON Table2.Item = Table1Items.Item 
  WHERE Completed != '0000-00-00' AND Completed &gt;= '2012-09-25' AND Completed &lt;= '2012-10-25' 
  GROUP BY WEEK(Completed)
  </code></pre>
  
  <p>I'm getting:</p>
  
  <pre><code>Completed  ActivityCount    CompletedTimestamp
  2012-09-25  300            2012-09-25 00:00:00
  2012-10-02  764            2012-10-02 00:00:00
  2012-10-08  379            2012-10-08 00:00:00
  2012-10-17  659            2012-10-17 00:00:00
  2012-10-22  382            2012-10-22 00:00:00
  </code></pre>
  
  <p>some are 7 days apart, others are 6 days apart, others are 5.... and one is 9?</p>
  
  <p>Why does it group the dates by such strange intervals instead of just 7 days?</p>