title: How to get the number of days between two dates using boost::date_time
body: <p>Are there any APIs available in boost::date_time to get the number of days between two dates that is also calendar specific?</p>
  
  <p>Example, Number of days between 2005/01/01 and 2006/12/31 would be 730 in a seven day calendar and would be 504 in a five day calendar.</p>