Class Schedule Conflicts (1 Viewer)

Hi

I am creating a student planning tool for class. right now i can add a class to my "cart" and drop just fine. But i need a way for the query to not run if there is a time conflict between the class i am trying to add and the classes already in the cart. I have tried playing with dcount but no luck so far.

So basically it must:

check hour of the day for conflict
check days for conflict
give error message that says which classes are conflicting

Any help is greatly appreciated thank you