title: Difference of value between two different times at the same date
body: <p>I have a dataframe <code>df</code> as below:</p>
  <pre><code>Datetime                Value
  2020-03-01 08:00:00      10
  2020-03-01 10:00:00      12
  2020-03-01 12:00:00      15
  2020-03-02 09:00:00       1
  2020-03-02 10:00:00       3
  2020-03-02 13:00:00       8
  2020-03-03 10:00:00      20
  2020-03-03 12:00:00      25
  2020-03-03 14:00:00      15
  </code></pre>
  <p>I would like to calculate the difference between the value on the first time of each date and the last time of each date (ignoring the value of other time within a date), so the result will be:</p>
  <pre><code>Datetime      Value_Difference
  2020-03-01          5
  2020-03-02          7
  2020-03-03         -5
  </code></pre>
  <p>I have been doing this using a for loop, but it is slow (as expected) when I have larger data. Any help will be appreciated.</p>