title: Calculate date time difference of two columns with null value in dataframe
body: <p>I had a dataframe like below:</p>
  
  <pre><code>    startdate   terminationdate
  0   1997-07-13  2004-09-29
  1   1999-07-26  2016-03-23
  2   2003-04-01  NaT
  3   2007-06-01  NaT
  4   2009-06-01  NaT
  
  </code></pre>
  
  <p>I would like to get the output to calculate the tenure in months. For null value in terminationdate, I would like to use current date to calculate.</p>
  
  <p>I tried the code below:</p>
  
  <pre><code>
  def tenure(df):
  
      if df['terminationdate'] != np.nan:
          tenure = (df['terminationdate'] - df['startdate'])/np.timedelta64(1, 'M')
  
      else:
          tenure = (datetime.datetime.now() - df['startdate'])/np.timedelta64(1, 'M')
      return tenure
  </code></pre>
  
  <p>The tenure of NaT value could not be calculated with above code.</p>