title: Drop rows of dataframe where dates are not the same
body: <p>I have two dataframes which should be of same length but are not.</p>
  
  <p>Its the stock prices of the last two days and the dataframes have the columns  close price, date and volume. </p>
  
  <p>I would now like to drop all rows of the dataframes where the dates are not equal so that I can plot and compare the data better. </p>
  
  <p>Can somebody help me with my issue?</p>
  
  <p>Thanks and kind regards</p>
  
  <pre><code>176   5338.8  2017-11-23T07:40:00.000000Z      81  
  177   5336.7  2017-11-23T07:45:00.000000Z     176  
  178   5342.8  2017-11-23T07:50:00.000000Z     109  
  179   5339.8  2017-11-23T07:55:00.000000Z     114  
  180   5342.8  2017-11-23T08:00:00.000000Z     285  
  181   5337.4  2017-11-23T08:05:00.000000Z     363  
  182   5333.9  2017-11-23T08:10:00.000000Z     174 
  </code></pre>
  
  <p>150 rows vs 180 rows. I would like to drop all rows where the dates are not equal to each other. </p>
  
  <pre><code>143   3549.3  2017-11-23T07:35:00.000000Z      14  
  144   3547.3  2017-11-23T07:40:00.000000Z       4  
  145   3548.3  2017-11-23T07:45:00.000000Z       8  
  146   3551.3  2017-11-23T07:50:00.000000Z       5  
  147   3548.3  2017-11-23T07:55:00.000000Z       9  
  148   3550.0  2017-11-23T08:00:00.000000Z      42  
  149   3548.0  2017-11-23T08:05:00.000000Z      89  
  150   3548.0  2017-11-23T08:10:00.000000Z      13  
  </code></pre>