title: Python Pandas Period Date difference is in * MonthEnds>, NaT, how to convert it into INT value
body: <p>I am trying to convert difference between two periods (having NaT values as well) into INT values.
  Getting error as: AttributeError: 'NatType' object has no attribute 'n'.</p>
  <pre><code>import pandas as pd
  date_start = pd.Period('10/2018',freq='M')
  df = pd.DataFrame({'id': [1, 2, 3, 4, 5, 6],
                    'date_end': ['2017-01', None , '2019-04','2018-06', 
                                 '2018-12', '2019-10']})
  df['date_end'] = pd.PeriodIndex(pd.to_datetime(df['date_end'], format='%Y-%m'),
                                  freq='M')
  
  df['diff'] = date_start - df['date_end']
  df['diff'].apply(lambda x: x.n)
  </code></pre>
  <p>What did I miss?</p>