title: Set time to default in openerp
body: <p>I have defined a field called 'datetime'. </p>
  
  <p>Here the time should be set to 08:00 by default and date we need to set manually.</p>
  
  <p>How can we do this ?</p>
  
  <p>I did something like this, here am setting date &amp; time by default. But unable to set only time.</p>
  
  <pre><code>_defaults = {
  'date_start': lambda self,cr,uid,context=None: fields.date.context_today(self,cr,uid,context) + " 02:30:00",  
  }
  </code></pre>
  
  <p>Please Suggest.</p>
  
  <p>Thanks.</p>