If it was legal, the person could say they smoked it three days ago. The test can't prove otherwise.
They can say the same thing in Ohio.
The crime is driving while impaired, not pot use. A test showing that they smoked pot at some time in the last month is not proof of impairment regardless of if it is illegal or not.
The test can prove they broke the law regarding using pot, but it does not prove they were impaired while driving. That gets proved with a sobriety check. Follow the finger with your eyes. Stand on one foot and touch your nose. All that stuff.