Views PHP Field - Comparing two date fields
You might need to add the $absolute parameter in your call to DateObject::difference(), in order to be able to get the sign:

$difference = $start_date-&gt;difference($end_date, 'days', FALSE);


According to the DateObject::difference documentation, the $absolute parameter:


  Indicate whether the absolute value of the difference should be
  returned or if the sign should be retained. Defaults to TRUE.