title: caluclate duration time without date in a pandas dataframe
body: <p>I've got a dataframe with a column (t1) giving the time someone usually goes to bed and a column (t2) giving the time someone usually gets up, looking like that:</p>
  <pre><code>ID       t1           t2
  p001   22:30:00    7:00:00
  ...
  p055   01:20:00    9:00:00
  ...
  </code></pre>
  <p>I want to create a new column giving the hours spent in bed, computed by those two times. A simple subtraction, like that</p>
  <pre><code>Df['hoursinbed'] = Df['t2']- Df['t1']
  </code></pre>
  <p>of course does not work since the starting time is usually on the day before.</p>
  <p>I've also tried this:</p>
  <pre><code>t1 = pd.to_datetime(Df['t1'])
  t2 = pd.to_datetime(Df['t2'])
  
  print (pd.Timedelta(t2-t1).seconds / 60.0)
  </code></pre>
  <p>But probably this works only if the start- and end-time includes a date?</p>
  <p>Does anyone have an idea how to solve this? TIA :)</p>