title: How to calculate the difference between two dates in OpenERP?
body: <p>In my custom module I am unable to calculate difference between two dates.</p>
  
  <p>My field is </p>
  
  <pre><code>'date_s':fields.datetime('Start Date'),
  'date_e':fields.datetime('End Date'),
  </code></pre>
  
  <p>I want to  calculate the difference  automatically  in my duration field:</p>
  
  <pre><code> 'Duration':fields.char('Duration'),
  </code></pre>