title: Get num of days between two dates, keep num of days the same when date is changed
body: <p>User wants to edit start date. Days between start date and end date must always be the same.</p>
  
  <p>For example</p>
  
  <p>Start Date is originally 28/07/2014 and End Date is 01/08/2014</p>
  
  <p>The dates are 4 days apart</p>
  
  <p>User changes start date to 01/06/2014</p>
  
  <p>End Date must now be 05/06/2014</p>
  
  <p>Having difficulty wrapping my head around this logic. </p>