Can the date of an alleged offense be changed after the plea hearing?

I got a fine for criminal damage to property at a bar. On the fine it says the date I allegedly committed the crime was April 1st, 2010. I go to court, plead not guilty, and asked the judge about the date and he confirmed that is when I allegedly did it. A few days after court, I get a piece of mail that wants me to pay restitution, but this doent states I committed the crime on April 9th, 2010. I have my trial on June 21st, 2010 and I want to be prepared. I have alibis for both dates, but I was wondering if I could get the fine dismissed or dropped since they can't pinpoint a date. I live in Wisconsin if that helps.