title: Find values from a column in a DF at very specific times for every unique date
body: <p>I asked this question and I got an answer which works for a general case with sequential and non missing data but not for my case specifically. I have a DF that looks as follows. </p>
  
  <pre><code>eventTime       MeteredEnergy Demand RunningHoursLamps 
  6/7/2018 0:00   67.728           64  1037.82
  6/7/2018 1:00   67.793           64  1038.82
  6/7/2018 2:00   67.857           64  1039.82
  6/7/2018 3:00   67.922           64  1040.82
  6/7/2018 4:00   67.987           64  1041.82
  6/7/2018 5:00                    64  1042.82
  6/7/2018 6:00                        1043.43
  6/7/2018 23:00  68.288
  6/8/2018 0:00   67.728           64  1037.82
  6/8/2018 23:00  67.793           64  1097.82
  </code></pre>
  
  <p>I need a DF that finds the difference between RunningHoursLamps values at hour 0 and hour 23 for each unique date in "eventTime" If data is missing for hour 0 or hour 23, the resultant DF can have NaN </p>
  
  <pre><code>Expected output
  
      Date        00:00       23:00       Difference 
      6/7/2018    1037.82     NaN         NaN
      6/8/2018    1037.82     1097.82     60
  </code></pre>