title: error in calculating minutes from timedelta objects in pandas dataframe
body: <p>I tried calculating minutes by subtracting two columns. But an error shows up which is " ValueError: unit abbreviation w/o a number".
  However, I tried similar operation on two different columns but it worked.</p>
  
  <p>I tried calculating arr_delay, but there was no error. But error came up when I tried calculating dep_delay. </p>
  
  <pre><code>data['arr_delay'] = (pd.to_timedelta(data.ATA) - pd.to_timedelta(data.STA)).dt.total_seconds()/60
  data['dep_delay'] = (pd.to_timedelta(data.ATD) - pd.to_timedelta(data.STD)).dt.total_seconds()/60
  </code></pre>
  
  <p>I was able to calculate arr_delay. But an error came up while calculating dep_delay which is :-" ValueError: unit abbreviation w/o a number "</p>
  
  <p><a href="https://i.stack.imgur.com/f7jha.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/f7jha.png" alt="Data Sample"></a></p>