title: pandas date column subtraction
body: <p>I have a pandas dataframe like this..</p>
  
  <pre><code>       created_time  reached_time
  2016-01-02 12:57:44      14:20:22
  2016-01-02 12:57:44      13:01:38
  2016-01-03 10:38:51      12:24:07
  2016-01-03 10:38:51      12:32:11
  2016-01-03 10:38:52      12:23:20
  2016-01-03 10:38:52      12:51:34
  2016-01-03 10:38:52      12:53:33
  2016-01-03 10:38:52      13:04:08
  2016-01-03 10:38:52      13:13:40
  </code></pre>
  
  <p>I want to subtract these two date columns and want to get <code>time</code></p>
  
  <p>I am doing following in python</p>
  
  <pre><code>speed['created_time'].dt.time - speed['reached_time']
  </code></pre>
  
  <p>But it gives me following error </p>
  
  <p><code>TypeError: ufunc subtract cannot use operands with types dtype('O') and dtype('&lt;m8[ns]')</code></p>
  
  <p>datatype of <code>created_time</code> is <code>object</code> and datatype of <code>reached_type</code> is <code>timedelta64[ns]</code></p>