title: calculate the difference of two timestamp columns
body: <p>I have a dataset like this:</p>
  <pre><code>data = pd.DataFrame({'order_date-time':['2017-09-13 08:59:02', '2017-06-28 11:52:20', '2018-05-18 10:25:53', '2017-08-01 18:38:42', '2017-08-10 21:48:40','2017-07-27 15:11:51',
                                     '2018-03-18 21:00:44','2017-08-05 16:59:05', '2017-08-05 16:59:05','2017-06-05 12:22:19'],
                  'delivery_date_time':['2017-09-20 23:43:48', '2017-07-13 20:39:29','2018-06-04 18:34:26','2017-08-09 21:26:33','2017-08-24 20:04:21','2017-08-31 20:19:52',
                                        '2018-03-28 21:57:44','2017-08-14 18:13:03','2017-08-14 18:13:03','2017-06-26 13:52:03']})
  </code></pre>
  <p>I want to calculate the time differences between these dates as the number of days and add it to the table as the delivery delay column. But I need to include both day and time for this calculation
  for example, if the difference is 7 days 14:44:46 we can round this to 7 days.</p>