First a remark: do not reference Fuel\Core directly, but use \Validation::forge(). It doesn't cause your issue, but it may cause extension and class reference issues.
the valid_date rule is defined in the Validation class itself, so it is very odd that it can't be found. What version of FuelPHP are you using? I just pasted your code in a test controller, and it runs find without issues...
I don't know netbeans, but I know you need to use some tricks to make netbeans aware of the existense of the aliased classes.
For phpstorm I use a dummy classes.php file in the root that defines all aliased classes, probably not entirely complete: http://bin.fuelphp.com/snippet/view/fb