Most Courts are very aware of the time marijuana stays in your system. If you test positive at your first visit they may use that as a baseline to determine future results. If, however, you try to state it is old use outside the time period generally recognized you may find yourself in trouble. Honesty is usually the best policy in those situations. If you tell your probation officer that you are likely to flunk the test because you used marijuana prior to the Court date you are likely going to ok. That being said, like every other profession, sometimes you run into someone that is incapable of using good reasoning skills.
You aren't the first pot smoker the drug counselors and probation officers have run across. Just let them know you may have been exposed to marijuana in the past but not since your arrest. If you test positive they will expect that it will decrease to zero on subsequent tests. Most DUI cases take months before you get on probation, so you should be aware that they might not believe you still have cannabanoids in your system since before your arrest.