title: 'NaTType' object has no attribute 'days'
body: <p>I have a column in my dataset which represents a date in ms and sometimes its values is <code>nan</code> (actually my columns is of type <code>str</code> and sometimes its valus is <code>'nan'</code>). I want to compute the epoch in days of this column. The problem is that when doing the difference of two dates:</p>
  
  <pre><code>(pd.to_datetime('now') - pd.to_datetime(np.nan)).days
  </code></pre>
  
  <p>if one is <code>nan</code> it is converted to <code>NaT</code> and the difference is of type <code>NaTType</code> which hasn't the attribute <code>days</code>.</p>
  
  <p>In my case I would like to have <code>nan</code> as a result.</p>
  
  <p>Other approach I have tried: <code>np.datetime64</code> cannot be used, since it cannot take as argument <code>nan</code>. My data cannot be converted to <code>int</code> since <code>int</code> doesn't have <code>nan</code>.</p>