title: Add months to a datetime column in pandas
body: <p>I have a dataframe df with 2 columns as below -</p>
  
  <pre><code>               START_DATE             MONTHS
  0              2015-03-21                240
  1              2015-03-21                240
  2              2015-03-21                240
  3              2015-03-21                240
  4              2015-03-21                240
  5              2015-01-01                120
  6              2017-01-01                240
  7                     NaN                NaN
  8                     NaN                NaN
  9                     NaN                NaN
  </code></pre>
  
  <p>The datatypes of the 2 columns are objects.</p>
  
  <pre><code>&gt;&gt;&gt; df.dtypes
  START_DATE    object
  MONTHS        object
  dtype: object
  </code></pre>
  
  <p>Now, I want to create a new column "Result" by adding df['START_DATE'] &amp; df['MONTHS']. So, I have done the below -</p>
  
  <pre><code>from dateutil.relativedelta import relativedelta  
  
  df['START_DATE'] = pd.to_datetime(df['START_DATE'])
  df['MONTHS'] = df['MONTHS'].astype(float)
  
  df['offset'] = df['MONTHS'].apply(lambda x: relativedelta(months=x))
  
  df['Result'] = df['START_DATE'] + df['offset'] 
  </code></pre>
  
  <p>Here, I get the below error -</p>
  
  <pre><code>TypeError: incompatible type [object] for a datetime/timedelta operation
  </code></pre>
  
  <p>Note: Wanted to convert df['Months'] to int but wouldn't work as the field had Nulls.</p>
  
  <p>Can you please give me some directions.Thanks.</p>