title: Pandas dataframe add integer columns into datetime columns
body: <p>I have a column named <code>transaction_date</code> which stores date, <code>1970-01-01</code> for example, and <code>payment_plan_days</code> stores the amount of days, 30, 70, any integer. </p>
  
  <p>How should I add <code>payment_plan_days</code> into <code>transaction_date</code> to create a new column as <code>membership_expire_date</code> ?</p>
  
  <p>I had tried with code below and it doesn't work since they are not the same dtype.</p>
  
  <pre><code>df_transactions.loc[(df_transactions['membership_expire_date'] == '19700101'), 'membership_expire_date'] = 
      df_transactions.loc[(df_transactions['membership_expire_date'] == '19700101'), 'transaction_date'] 
      + df_transactions.loc[(df_transactions['membership_expire_date'] == '19700101'), 'payment_plan_days']
  </code></pre>