title: Time Difference returns NULL
body: <p>I am trying to get the time for a contract to run out from my database. I now have this query, but I cannot seem to figure out why I get NULL for the to_expire field. The query runs fine, and produces the expected results except for the calculated timediff. cs_start and cs_end are date fields</p>
  
  <pre><code>  SELECT cs_start, cs_end, cs_status, 
   TIMEDIFF(date(now()),  cs_start) as to_expire,
   GROUP_CONCAT(is_name SEPARATOR '&lt;br /&gt; ') as partners 
    FROM `contracttoinstitute` 
    JOIN `institutes`
      ON is_id = ctt_institute_id
    JOIN `contracts` 
      on ctt_contract_id  = cs_id
   WHERE ctt_contract_id 
      IN (
        select ctt_contract_id 
          from `contracttoinstitute`
         where ctt_institute_id = 1 
         )
     AND ctt_institute_id &lt;&gt; 1 
    GROUP BY ctt_contract_id
    ORDER BY cs_id DESC
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code> cs_start cs_end cs_status to_expire partners 
  2013-11-06  2014-01-01  signed  NULL    KATHOLIEKE HOG   
  ...
  </code></pre>
  
  <p>As was hoping to get some value. Not sure what timediff SHOULD give; First time I try to use it. (I am looking for a value to sort by, in order to determine whether a contract is about to lapse)</p>