Hi ,
I am assuming that the days are inclusive , thus if the check-in is on 26-06-2016 , and the check-out is also on the same day , the guest still has to pay for one day.
Thus between 26-06-2016 and 06-07-2016 , there would be 11 days and not 10.
That is the reason there is a +1 in the formula. If you think this is wrong , remove this +1 from the formula.
Narayan