title: Subtract two columns in pandas dataframe
body: <p>I have two columns in pandas dataframe that represent hour of the day in 24 hour format, i.e., <code>18:00:00</code>. Both of them are in object datatype and I want to find the difference in hours of the two columns. For eg. difference between <code>18:00:00</code> and <code>17:00:00</code> should come out as 1. I tried using <code>to_timedelta</code> function but it returns 'no units specified' error even after I specify unit as 'h'.</p>
  
  <pre><code>d = {'col1': ['18:00:00', '19:00:00'], 'col2': ['17:00:00', '17:00:00']}
  df = pd.DataFrame(data=d)
  df
  
  df['col1']-df['col2']
  </code></pre>
  
  <p>Can anyone assist in this? Thanks in Advance.</p>