I get from your code, that you might have assumed that the end date is the problem.
It's the start date (Oldest) that skips the first day.
e.g.
$WADbSearch1->addComparisonFromList("VisitorDate","S_VisitorDate","AND",">=",2);
Or same with "Edit":
$WADbSearch1->addComparisonFromEdit("VisitorDate","S_VisitorDate","AND",">=",2);
If I could just get a "minus one day (-1)" in there somewhere it might fix it, because otherwise it works perfectly.