title: Pandas Dataframe convert string to data without time
body: <p>I have a Pandas Dataframe df:</p>
  
  <pre><code>a     date
  1     2014-06-29 00:00:00
  </code></pre>
  
  <p>df.types return:</p>
  
  <pre><code> a    object
   date object
  </code></pre>
  
  <p>I want convert column data to data without time but:</p>
  
  <pre><code>df['date']=df['date'].astype('datetime64[s]')
  </code></pre>
  
  <p>return:</p>
  
  <pre><code>a   date
  1   2014-06-28 22:00:00
  </code></pre>
  
  <p>df.types return:</p>
  
  <pre><code> a    object
   date datetime64[ns]
  </code></pre>
  
  <p>But value is wrong.</p>
  
  <p>I'd have:</p>
  
  <pre><code>a     date
  1     2014-06-29
  </code></pre>
  
  <p>or: </p>
  
  <pre><code>a     date
  1     2014-06-29 00:00:00
  </code></pre>