Hi Guys,
I've seen a lot of people asking this here, so I've taken some code that was already done and tweaked it to my needs.
Hope this helps someone else.
http://www.meartechnology.co.uk/blog/2016/09/16/infopath-calculate-business-days-between-2-dates-including-leap-year/
If you have any idea to improve my solution, your help will be much appreciated.