This is the very reason I'm against testing for pot. It serves no purpose other then to catch people in a gotcha moment. Unlike other drugs and even alcohol, pot sticks around so you don't have to be currently high to test positive. The rest of the drugs have a higher chance of being detected if under the influence at the time of being tested.
Not really sure there is a way around this. You might want to contact a lawyer who is familial with this sort of thing. You can't really prove you hadn't used in quite some time unless they test your hair and most companies will not spring for that level of testing.