If you smoke a joint 3 weeks before you drive a car, you arent "under the influence". Anyone that says otherwise is a dumbass or has never smoked.
Just because it is in your system a month later, doesnt mean you are impaired. It isnt like alcohol. Hell, he couldve smoked 100 joints 12 hours before and not have any effects. Just dumb stupid ignorant laws. Smoking all day isnt like drinking all day . You dont wake up the next day "off".