drug tests at work

i recently got hired, and i have been working at this new job for a week now. they didn't give me a drug test, the only thing they did was make me sign a paper stating that they will not tolerate drug and alcohol abuse on the work premises. which to me it sounds like you can do whatever you want on your own time but just dont bring it to work. well the only catch to that, that i didn't understand was in the paper it also stated that you have 5 days to tell your supervisor prior to a drug conviction on the work premises. i really didnt understand that part in that form but i was just wondering if any of you might have some insight on this. any help will be appreciated, i just really want to know cause i love to smoke bud and i dont want to ask anyone there cause i like to keep that sort of business to myself. but again if anyone has experienced anything like this please comment me back, thanks.