Urgent! Please help on J1 waiv

When I am filling the Data Sheet of J1 waiver, there has a question about"Give an explanation for any period of time in the U.S> not covered by IAP-66". I did stay in U.S. for 30 days after my IAP-66 expired. Would you please tell me how I explain? Is 30 days legal? I am so worried about it. My University said I could leave up to 30 days later after my IAP-66 expired.