title: get next value in list Pandas
body: <p>I have a list of unique dates in chronological order.<br>
  I have a dataframe with dates in it. I want to use the list of dates in the dataframe to get the NEXT date in the list (find the date in dataframe in the list, return the date to the right of it ( next chronological date).</p>
  
  <p>Any ideas?</p>