title: How to find empty datatime in Pandas
body: <p>I have a Pandas dataframe which has a column <code>date_closed</code> of type <code>datetime64[ns]</code>. When I find the unique values there are some NaT there. How can I only filter the rows whose date_closed in not NaT</p>
  
  <pre><code>df['date_closes'].unique()
  </code></pre>
  
  <p>btw, what does NaT means? is it Null? 
  When I value_counts() , the NaT's does not appear! Why?</p>
  
  <pre><code>df['date_closed'].value_counts()
  </code></pre>