I need help writing a formula...

Hi all, I need some help writing an if/then formula... Basically, what I need it to do is, when I select an inventory item from a list, say IF (current, associated job set date) is BETWEEN (any other job set date and strike date) AND/OR (current job strike date) is BETWEEN (any other job set date and strike date) THEN show me the (total quantity of that inventory item owned) MINUS (total quantity of that inventory item proposed for any jobs matching the above date criteria), AND IF (current, associated job set date) IS NOT BETWEEN (any other job set date and strike date) AND (current, associated job strike date) IS NOT BETWEEN (any other job set date and strike date) THEN show me the (total quantity of that inventory item owned). 

Thank you in advance! I realize this is a pretty complex one, and I just don't know the QB code well enough to do this.