Is my passport valid?

Here's the situation. I live in Toronto, Ontario, Canada and I was planning to go to Detroit, Michigan for the day on June 27, 2011. My passport expires June 27, 2011. I've been to the USA Embassy website and it states that my passport only needs to be valid for the duration of my stay and it's a common misconception that it needs to be valid for an additional 6 months. Now my problem is, is my passport invalid ON June 27 or does it become invalid at the END of June 27?