Brazil Visa - What does it mean exactly 30 days? When do I have to leave?

When I entered Brazil from Colombia, I got a 30-day visa (entry stamp) in my pport. I got it on the 12th of March. Do I start counting the 30 days on the 12th, or on the 13th? Meaning, do I have to leave Brazil on April 10th, or 11th? What happens if I leave a day late?