i think the "age in working days" in the helpdesk explorer is not calculating properly in V9.1.6

I have set up the master site with the working hours on Monday to Friday. Every staff member has been assigned to this site and in turn this site has been setup as such in the adminstrator menu.

I have some jobs logged on 28th April showing as 5 days and then a couple of jobs which show 4 working days - all where logged within the working hours for the master site.

Is anyone else using this field and have you found the same