title: Replace date with NaT in Pandas dataframe
body: <p>I have got a dataframe with a column of datetime64 type. In this column there are several rows with dates as 1999-09-09 23:59:59 where as they should have actually been represented as missing dates NaT. Somebody just decided to use this particular date to represent the missing data. Now I want these dates to be replaced as NaT (the missing date type for Pandas). </p>
  
  <p>Also if I perform operation on this column with NaTs, like </p>
  
  <pre><code>df['date'] - df['column with missing date']
  </code></pre>
  
  <p>Does Pandas ignore the missing dates and maintain NaT for those rows or will it throw an error some thing like Null pointer exception in Java.</p>