Can they still charge me even though it has been past 77 days?

I was arrested for suspicion of operating under the influence (marijuana) and speeding. I was taken to the hospital for blood and urine test then taken to jail for 2 hour and bonded out. I had a court date to see the magistrate 2 weeks later. All charges were dropped. My bond was returned. This was on December 14, 201. On March 14, 2015, I received a letter from the prosecutor saying that they would be charging me with operating with the presence of drugs in my system. I had not smoke within 5 hours of driving but told the officer I do smoke and would not have a clean urine.